Most of sole behind cat's third claw (5)
I believe the answer is:
talon
'claw' is the definition.
(I know that talon is a type of claw)
'most of sole behind cat's third' is the wordplay.
'most of' means to remove the last letter.
'sole' becomes 'alone' (synonyms).
'behind' is a charade indicator (letters next to each other).
'cat's third' becomes 't' (3rd letter of 'cat').
'alone' with its final letter taken off is 'alon'.
'alon' after 't' is 'TALON'.
(Other definitions for talon that I've seen before include "hunting hook" , "Hooker" , "Eg, an eagle's claw" , "Prey-bird's claw" , "The stock" .)