Nervous of attack by a force (6)
I believe the answer is:
afraid
'nervous' is the definition.
(synonyms)
'attack by a force' is the wordplay.
'attack' becomes 'raid' (raiding is a kind of attacking).
'by' is a charade indicator (letters next to each other).
'force' becomes 'F' (symbol used in Physics e.g. f=ma).
'a'+'f'='af'
'raid' put after 'af' is 'AFRAID'.
'of' acts as a link.
(Other definitions for afraid that I've seen before include "Nervous" , "Alarmed, fearful" , "Frightened" , "with butterflies" , "Scared - politely regretful" .)