Clarkson completed a one-month loan move to Brentford on 15 September 2011 and made his debut in the away win at Oldham Athletic on 24 September. He scored his first goal for the club away to Carlisle United on 8 October 2011, in a 2–2 draw. Clarkson gave Brentford the lead within the first 10 minutes, but on 63 minutes, he scored an own goal when he stuck a leg out from a cross and unfortunately put the ball into his own net to give Carlisle a 2–1 lead. He made amends by assisting for Toumani Diagouraga to level the match two minutes from the end.