Part of saw's also extremely tough (5)
I believe the answer is:
tooth
'part of saw's' is the definition.
('tooth' is part of 'saw')
'also extremely tough' is the wordplay.
'also' becomes 'too' (synonyms).
'extremely' means to remove the middle letters (only the extremes of the word are used).
'tough' with its centre removed is 'th'.
'too'+'th'='TOOTH'
(Other definitions for tooth that I've seen before include "Taste" , "In which the old are long" , "Canine or incisor?" , "1 across" , "Incisor, e.g." .)