Tune just lacks force (3)
I believe the answer is:
air
'tune' is the definition.
('air' can be a synonym of 'tune')
'just lacks force' is the wordplay.
'just' becomes 'fair' ('fair' is associated in meaning with 'just').
'lacks' suggests deleting specific letters.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'fair' with 'f' removed is 'AIR'.
(Other definitions for air that I've seen before include "Ventilate a tune?" , "'Tune, melody (3)'" , "Musical composition" , "An impressive manner or tune" , "Broadcast - song" .)