Tweak soft part of foot (5)
I believe the answer is:
pinch
'tweak' is the definition.
('pinch' can be a synonym of 'tweak')
'soft part of foot' is the wordplay.
'soft' becomes 'p' (musical abbreviation for 'piano').
'part of foot' becomes 'inch' ('inch' is part of 'foot').
'p'+'inch'='PINCH'
(Other definitions for pinch that I've seen before include "appropriate" , "amount of salt?" , "Pilfer" , "Economise - steal" , "Squeeze between the fingers" .)