Force isn't weak (5)
I believe the answer is:
faint
'weak' is the definition.
(I've seen this before)
'force isn't' is the wordplay.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'isn't' becomes 'aint' (I've seen this before).
'f'+'aint'='FAINT'
(Other definitions for faint that I've seen before include "(Of a chance) slight" , "Indistinct - pass out" , "hard to hear" , "woozy" , "Barely perceptible" .)