Let in many in the entrance (5)
I believe the answer is:
admit
'let in' is the definition.
(synonyms)
'many in the entrance' is the wordplay.
'many' becomes 'm'.
'in' indicates putting letters inside.
'the entrance' becomes 'adit' (I've seen this before).
'm' inserted into 'adit' is 'ADMIT'.
(Other definitions for admit that I've seen before include "Confess - acknowledge" , "Allow access" , "Let in - concede" , "Usher in" , "Agree to the truth" .)