Illegal-sounding object for a thief (6)
I believe the answer is:
bandit
'a thief' is the definition.
(bandit is a kind of thief)
'illegal-sounding object' is the wordplay.
'illegal' becomes 'banned' ('banned' can be similar in meaning to 'illegal').
'sounding' shows a homophone (sound like).
'object' becomes 'it' (pronoun for an inanimate object**).
'banned' is a homophone of 'band'.
'band'+'it'='BANDIT'
'for' acts as a link.
(Other definitions for bandit that I've seen before include "Outlaw, brigand" , "Outlawed robber" , "Member of a robber gang" , "Enemy aircraft" , "hood" .)