Possibly flat pastry mistake over putting fruit in (4-1-5)
I believe the answer is:
pied-a-terre
'possibly flat' is the definition.
'pied a terre' can be an answer for 'flat' (I've seen this before). I am not sure about the 'possibly' bit.
'pastry mistake over putting fruit in' is the wordplay.
'pastry' becomes 'pie' (pie is a kind of pastry).
'mistake' becomes 'err' (to err is to make a mistake or blunder).
'over putting fruit' becomes 'date' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'err' put into 'date' is 'daterre'.
'pie'+'daterre'='PIED-A-TERRE'
Can you help me to learn more?
(Other definitions for pied-a-terre that I've seen before include "Small secondary flat or house" , "Dwelling for occasional use" , "Lodging kept for occasional use (French)" , "Small home kept for occasional use" , "bijou accommodation" .)