Urchin lad wandering in gloom (7)
I believe the answer is:
mudlark
'urchin' is the definition.
(thesaurus)
'lad wandering in gloom' is the wordplay.
'wandering' indicates an anagram.
'in' means one lot of letters goes inside another.
'gloom' becomes 'murk' (I've seen this before).
'lad' anagrammed gives 'dla'.
'dla' put within 'murk' is 'MUDLARK'.
(Other definitions for mudlark that I've seen before include "flyer" , "Scavenger" , "One scavenging beside river" .)