Frenchman has unusual date 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)
'frenchman has unusual date' is the wordplay.
'frenchman' becomes 'pierre' (common French name).
'has' is an insertion indicator.
'unusual' is an anagram indicator.
'date' anagrammed gives 'edat'.
'pierre' going around 'edat' is 'PIED-A-TERRE'.
'in' acts as a link.
(Other definitions for pied a terre that I've seen before include "1 across [MAISONETTE], possibly" , "second home" , "temporary home" , "city flat" , "accommodation" .)