Tooth is able to lose first line (6)
I believe the answer is:
canine
'tooth' is the definition.
(I know that canine is a type of tooth)
'able to lose first line' is the wordplay.
'able' becomes 'can'.
'to lose first' means to remove the first letter (I've seen 'losing first' mean this).
'line' with its first letter taken off is 'ine'.
'can'+'ine'='CANINE'
'is' acts as a link.
(Other definitions for canine that I've seen before include "Boxer" , "Doggy; tooth" , "like a setter" , "Dog-like" , "A tooth; of our best friend" .)