Provided Force with unknown suspect (4)
I believe the answer is:
iffy
'suspect' is the definition.
(I've seen this before)
'provided force with unknown' is the wordplay.
'provided' becomes 'if'.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'with' says to put letters next to each other.
'unknown' becomes 'y' (algebra).
'if'+'f'+'y'='IFFY'
(Other definitions for iffy that I've seen before include "Not certain, doubtful" , "not kosher" , "Not to be trusted" , "Doubtful, not definite" , "Questionable - uncertain" .)