Trailing 9-3 at the break, the hosts came out firing and scored 16 unanswered points in difficult conditions – thanks predominantly to a dominant performance from the Clevedon pack.
Alex Giltrow touched down for the only try of the game five minutes from time – while Mike Taylor signed off a 14-point haul with last-play drop goal, to ensure Thornbury left empty-handed.
In heavy conditions at Clevedon School, the hosts started in positive fashion and went close to opening the scoring on five minutes when Rory Biggins exploited space on the blindside – but the centre was unable to get an offload away before being bundled into touch.
What followed was a sloppy 20 minute spell from Clevedon, which allowed Tim Stephens to kick Thornbury into a 6-0 lead.
Giltrow was then adjudged to have been held-up just short of the line from a close range pick – and a disgruntled Clevedon side were quickly penalised for airing their disapproval to the referee.
Stephens added a third penalty from 30 metres before Taylor replied in kind on the half-hour mark - but ill-discipline and poor decision making ensured the hosts trailed at half-time.
The second-half began with Clevedon very much in the ascendancy – and they were rewarded for their early dominance five minutes in, when Taylor notched his second penalty of the afternoon.
Now on the front foot, the Clevedon forward pack went to work at scrum-time - with props Giltrow, and man of the match Brian Largenton to the fore - and were perhaps unlucky not to be awarded a penalty try with the Thornbury eight beginning to buckle.
Taylor was twice off target from the tee as Clevedon edged closer, but made amends on 60 minutes to level the scores at 9-9.
Minutes later a powerful driving maul saw Clevedon held-up two metres short, before wasteful play from the backline saw another chance go begging – and when a wheeled five-metre scrum saw the referee turn the ball over, Clevedon could have been forgiven for thinking it wasn’t going to be their day.
But from the resulting scrum, another destructive shove from the Clevedon pack drove Thornbury back behind their own line – and with the ball spilled loose, Giltrow pounced to give his side the lead for the first time in the game.
Taylor added the conversion, before rounding off proceedings with an expertly taken drop-goal to deny Thornbury a losing bonus-point and seal a fourth successive win for Clevedon.
Clevedon travel to Winscombe next Saturday in the semi-final of the Bristol Combination Vase.