Occasion entrants need to attend in Lycra tops (6)
I believe the answer is:
entail
'occasion' is the definition.
(I've seen this before)
'entrants need to attend in lycra tops' is the wordplay.
'tops' says to take the initial letters.
The initial letters of 'entrants need to attend in lycra' is 'ENTAIL'.
(Other definitions for entail that I've seen before include "Involve or require" , "Involve (something) as an inevitable consequence" , "Occasion" , "settlement of estate" , "Have as a logical consequence" .)