Go to the races after a knock on the head (6)
I believe the answer is:
attend
'go to' is the definition.
(synonyms)
'the races after a knock on the head' is the wordplay.
'the races' becomes 'tt' (Isle of Man Tourist Trophy races).
'after' is a charade indicator (letters next to each other).
'knock on the head' becomes 'end' (the knock something on the head is to finish it).
'tt' after 'a' is 'att'.
'att'+'end'='ATTEND'
(Other definitions for attend that I've seen before include "Go regularly to (school)" , "take heed" , "Listen carefully" , "Be present or appear at" , "Completed" .)