Unknown number visiting European city retire (4,2)
I believe the answer is:
turn in
'retire' is the definition.
(I know that retire can be written as turn in)
'unknown number visiting european city' is the wordplay.
'unknown number' becomes 'n'.
'visiting' means one lot of letters goes inside another.
'european city' becomes 'Turin' (Italian city).
'n' inserted within 'turin' is 'TURN-IN'.
(Other definitions for turn in that I've seen before include "Informally, go to bed or hand over" , "Surrender" , "Go to bed, changing direction" , "Go to bed (informal)" , "Prepare for sleep" .)