Nigel gets beaten up — he nicked someone's marbles (5)
I believe the answer is:
elgin
'he nicked someone's marbles' is the definition.
(as in the Elgin marbles)
'nigel gets beaten up' is the wordplay.
'gets beaten up' is an anagram indicator.
'nigel' anagrammed gives 'ELGIN'.
(Other definitions for elgin that I've seen before include "Scottish town (with marbles?)" , "Scottish place" , "One carrying off temple sections" , "Scottish earl who took statues from the Parthenon" , "The -- Marbles (British Museum)" .)