French city that is besieged by heartless lads (5)
I believe the answer is:
lyons
'french city' is the definition.
(I've seen this before)
'that is besieged by heartless lads' is the wordplay.
'that' becomes 'yon' (archaic term meaning 'that').
'is besieged by' is an insertion indicator (I've seen this in other clues).
'heartless' means to remove the middle letters.
'lads' with its centre taken out is 'ls'.
'yon' inserted into 'ls' is 'LYONS'.
(Other definitions for lyons that I've seen before include "Famous caterer" , "anglicised French place name" , "City of central France" , "old-world tea shops" , "Capital of Rhone department of France" .)