Managing to bite back, holding with the tooth (6)
I believe the answer is:
coping
'managing' is the definition.
('cope' can be a synonym of 'manage')
'to bite back holding with the tooth' is the wordplay.
'to bite' means one lot of letters goes inside another (to have a bite can mean to consume).
'back holding with' becomes 'pin' (I can't justify this - if you can you should give a lot more credence to this answer).
'the tooth' becomes 'cog' (cog is a kind of tooth).
'pin' put into 'cog' is 'COPING'.
Can you help me to learn more?
(Other definitions for coping that I've seen before include "wall feature" , "Capping along the top of a stone wall" , "Managing, getting by" , "Doing well" , "sloping off course" .)