Force is not hard to see (5)
I believe the answer is:
faint
'hard to see' is the definition.
(I've seen this in another clue)
'force is not' is the wordplay.
'force' becomes 'F' (symbol used in Physics e.g. f=ma).
'is not' becomes 'aint' (I've seen this before).
'f'+'aint'='FAINT'
(Other definitions for faint that I've seen before include "dull" , "(Of a chance) slight" , "Swoon, pass out" , "Swoon; indistinct" , "Swoon - pale" .)