Allowed in before the entrance (5)
I believe the answer is:
inlet
'the entrance' is the definition.
(I've seen this before)
'allowed in before' is the wordplay.
'allowed' becomes 'let' (to let is to allow or permit).
'before' says to put letters next to each other.
'let' after 'in' is 'INLET'.
(Other definitions for inlet that I've seen before include "Place of entry" , "Means of entry" , "Arm of lake, river" , "Narrow arm of water" , "Sound" .)