Frowned as force decreased (8)
I believe the answer is:
glowered
'frowned' is the definition.
('glower' can be a synonym of 'frown')
'force decreased' is the wordplay.
'force' becomes 'g' (short for gravity).
'decreased' becomes 'lowered' (lowering is a kind of decreasing).
'g'+'lowered'='GLOWERED'
'as' is the link.
(Other definitions for glowered that I've seen before include "Looked furious" , "Stared angrily" , "Scowled" , "Showed disapproval" .)