Tart on romantic rendezvous admitting to sin in small flat (4-1-5)
I believe the answer is:
pied-a-terre
'small flat' is the definition.
(French term for a small flat)
'tart on romantic rendezvous admitting to sin' is the wordplay.
'tart' becomes 'pie' (I've seen this before).
'on' says to put letters next to each other.
'romantic rendezvous' becomes 'date' (romantic meeting).
'admitting' is an insertion indicator (admit can mean to welcome in or accept).
'to sin' becomes 'err' (to sin is to err morally).
'date' enclosing 'err' is 'daterre'.
'pie'+'daterre'='PIED-A-TERRE'
'in' acts as a link.
(Other definitions for pied-a-terre that I've seen before include "Lodging kept for occasional use (French)" , "Small secondary flat or house" , "bijou accommodation" , "Small home kept for occasional use" , "Dwelling for occasional use" .)