Measure force returning as well (4)
I believe the answer is:
foot
'measure' is the definition.
(foot is a kind of measure)
'force returning as well' is the wordplay.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'returning' shows that the letters should be reversed in order.
'as well' becomes 'too' (both can mean 'also').
'too' back-to-front is 'oot'.
'f'+'oot'='FOOT'
(Other definitions for foot that I've seen before include "Infantry - unit of linear measure" , "support" , "Metrical unit; pay (a bill)" , "Unit of length" , "Twelve inches" .)