Very many men put off about ecstasy (7)
I believe the answer is:
umpteen
'very many' is the definition.
(I've seen this before)
'men put off about ecstasy' is the wordplay.
'off' is an anagram indicator.
'about' is an insertion indicator.
'ecstasy' becomes 'E' (informal term for the drug).
'men'+'put'='menput'
'menput' with letters rearranged gives 'umpten'.
'umpten' placed around 'e' is 'UMPTEEN'.
(Other definitions for umpteen that I've seen before include "Lots of things on pet menu" , "Indefinitely many" , "Numerous" , "Unspecified large number" , "Great in number but unspecified" .)