Available gents, perhaps one dropping out (2,3)
I believe the answer is:
to let
'available' is the definition.
(I've seen this in another clue)
'gents perhaps one dropping out' is the wordplay.
'gents perhaps' becomes 'toilet'.
'one' becomes 'i' (Roman numeral).
'dropping out' is a deletion indicator.
'toilet' with 'i' removed is 'TO LET'.
(Other definitions for to let that I've seen before include "Estate agent's sign" , "open to offers" , "Available for a tenant" , "Sign on property available to rent" , "Sign of the property market" .)