One is out of shape lacking force (4)
I believe the answer is:
unit
'one' is the definition.
('unit' can be a synonym of 'one')
'out of shape lacking force' is the wordplay.
'out of shape' becomes 'unfit' (I've seen this before).
'lacking' suggests deleting specific letters.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'unfit' with 'f' removed is 'UNIT'.
'is' is the link.
(Other definitions for unit that I've seen before include "- - trust" , "Single item; one" , "Individual item" , "Single thing or person" , "Kitchen fitment" .)