Try to introduce new restraint (4)
I believe the answer is:
bind
'restraint' is the definition.
(bind can mean a restraint)
'try to introduce new' is the wordplay.
'try' becomes 'bid' (bid is a kind of try).
'to introduce' is an insertion indicator (to introduce to can mean to insert or include).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'bid' going around 'n' is 'BIND'.
(Other definitions for bind that I've seen before include "Impose a contractual obligation on (someone)" , "Senior figure" , "difficulty" , "difficult situation" , "Join or fasten together" .)