Put restrictions on gang criminal (4-5)
I believe the answer is:
ring-fence
'put restrictions on' is the definition.
(I've seen this before)
'gang criminal' is the wordplay.
'gang' becomes 'ring' (synonyms).
'criminal' becomes 'fence' (someone who handles stolen goods).
'ring'+'fence'='RING-FENCE'
(Other definitions for ring-fence that I've seen before include "Complete barrier" , "estate security" , "Guarantee funding" , "Protect or guarantee - with circular hedge?" , "Enclose with a barrier" .)