Take part in a series of motorbike time trials, then stop (6)
I believe the answer is:
attend
'take part in' is the definition.
(I've seen this before)
'a series of motorbike time trials then stop' is the wordplay.
'series of motorbike time trials' becomes 'TT' (Isle of Man Tourist Trophy races).
'then' means one lot of letters go next to another.
'stop' becomes 'end' (synonyms).
'a'+'tt'+'end'='ATTEND'
(Other definitions for attend that I've seen before include "Escort (someone)" , "notice" , "Follow" , "Being 16 [LISTEN]" , "Be there ? pay attention" .)