Cog, for example, excessively tough on the outside (5)
I believe the answer is:
tooth
'cog for example' is the definition.
(cog is a kind of tooth)
'excessively tough on the outside' is the wordplay.
'excessively' becomes 'too' (synonyms).
'on the outside' suggests removing the centre.
'tough' with its centre taken out is 'th'.
'too'+'th'='TOOTH'
(Other definitions for tooth that I've seen before include "One needed to chew" , "It erupts" , "Projection of a gear wheel" , "Canine, for example" , "Fang; cog" .)