Self-assertive dog biting part of leg (7)
I believe the answer is:
pushing
'self-assertive' is the definition.
(thesaurus)
'dog biting part of leg' is the wordplay.
'dog' becomes 'pug' (pug is a kind of dog**).
'biting' is an insertion indicator (I've seen this in other clues).
'part of leg' becomes 'shin' ('shin' is part of 'leg').
'pug' enclosing 'shin' is 'PUSHING'.
(Other definitions for pushing that I've seen before include "Enterprising" , "Pressing against" , "Shoving" .)