Bats heading off in a great crowd (4)
I believe the answer is:
army
'a great crowd' is the definition.
'bats heading off' is the wordplay.
'bats' becomes 'barmy' ('barmy' can be a synonym of 'bats').
'heading off' means to remove the first letter.
'barmy' with its first letter taken away is 'ARMY'.
'in' acts as a link.
(Other definitions for army that I've seen before include "Large host" , "Military land force" , "A defence force" , "Military unit" , "See 17" .)