Perhaps canine is overly tough on the outside (5)
I believe the answer is:
tooth
'perhaps canine' is the definition.
'overly tough on the outside' is the wordplay.
'overly' becomes 'too' ('too' can be a synonym of 'overly').
'on the outside' suggests removing the centre.
'tough' with its middle removed is 'th'.
'too'+'th'='TOOTH'
'is' acts as a link.
(Other definitions for tooth that I've seen before include "Jag on the edge of a saw" , "Projection on comb, saw or wheel" , "Eg, molar" , "Let it stand" , "Nipper" .)