Sign on apartment maybe one removed from gents? (2,3)
I believe the answer is:
to let
'sign on apartment maybe' is the definition.
(an apartment might be let out)
'one removed from gents?' is the wordplay.
'one' becomes 'i' (Roman numeral).
'removed from' is a deletion indicator.
'gents?' becomes 'toilet' (men's toilet).
'toilet' with 'i' taken away is 'TO LET'.
(Other definitions for to let that I've seen before include "Notice of vacancy" , "Sign of the property market" , "Sign on property available to rent" , "Available for renting" , "Estate agent's sign" .)