Tweak soft part of foot (5)
I believe the answer is:
Here is my best explanation:
'tweak' is the definition.
('pinch' can be a synonym of 'tweak')
'soft part of foot' is the subsidiary indication.
'soft' becomes 'p'.
'part of foot' becomes 'inch' ('inch' is part of 'foot').
(Other definitions for pinch that I've seen before include "Squeeze between the fingers", "Tweak", "Grip sharply; steal", "Grip tightly; steal (colloq.)", "Economise - steal", "Steal - a little salt, for example", "Nip - purloin", "Steal (informal); squeeze" and "Steal (colloq.); one felt by the poor".)