Being insane mostly helps pub staff (8)
I believe the answer is:
barmaids
'pub staff' is the definition.
(I've seen this before)
'being insane mostly helps' is the wordplay.
'being insane' becomes 'barmy' ('barmy' can be similar in meaning to 'insane').
'mostly' means to remove the last letter.
'helps' becomes 'aids' ('aid' can be a synonym of 'help').
'barmy' with its last letter taken off is 'barm'.
'barm'+'aids'='BARMAIDS'
(Other definitions for barmaids that I've seen before include "pub staff" , "in local employment" .)