Gentleman went undercover since stealing fifty (7)
I believe the answer is:
hidalgo
'gentleman' is the definition.
(I've seen this before)
'went undercover since stealing fifty' is the wordplay.
'went undercover' becomes 'hid' (I've seen this in another clue).
'since' becomes 'ago' (I've seen this in another clue).
'stealing' means one lot of letters goes inside another.
'fifty' becomes 'l' (Roman numeral).
'ago' enclosing 'l' is 'algo'.
'hid'+'algo'='HIDALGO'
(Other definitions for hidalgo that I've seen before include "Pachuca is capital of this Mexican state" , "Spanish fellow" , "Spanish gentleman" , "Spanish nobleman" , "foreign gentleman" .)