Force isn't commonly weak (5)
I believe the answer is:
faint
'weak' is the definition.
(I've seen this before)
'force isn't commonly' is the wordplay.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'isn't commonly' becomes 'aint' (I've seen this in other clues).
'f'+'aint'='FAINT'
(Other definitions for faint that I've seen before include "dull" , "woozy" , "Swoon, lose consciousness" , "Swoon; indistinct" , "Dim -- swoon" .)