Getting by with fix for breaking tooth (6)
I believe the answer is:
coping
'getting by' is the definition.
('cope' can be a synonym of 'get by')
'fix for breaking tooth' is the wordplay.
'fix' becomes 'pin' (to pin is to fix in place).
'for breaking' indicates putting letters inside (inserted letters break their way in).
'tooth' becomes 'cog' (cog is a kind of tooth).
'pin' inserted inside 'cog' is 'COPING'.
'with' is the link.
(Other definitions for coping that I've seen before include "Doing well" , "Managing - top course of a wall" , "sloping off course" , "wall feature" , "Capping along the top of a stone wall" .)