Mostly professional gang in pre-flood operation? (7)
I believe the answer is:
pairing
'pre-flood operation?' is the definition.
(Noah paired up animals before the biblical Flood)
'mostly professional gang in' is the wordplay.
'mostly' means to remove the last letter.
'professional' becomes 'paid' ('paid' can be similar in meaning to 'professional').
'gang in' becomes 'ring' ('ring' can be a synonym of 'gang'. I am not sure about the 'in' bit.**).
'paid' with its last letter removed is 'pai'.
'pai'+'ring'='PAIRING'
(Other definitions for pairing that I've seen before include "outcome of cup draw" , "Twosome" , "Couple" , "parliamentary procedure" .)