Island force caught up in attack: be shocked (4,1,3)
I believe the answer is:
have a fit
'be shocked' is the definition.
('have a fit' can mean to react badly to news)
'island force caught up in attack' is the wordplay.
'island' becomes 'i'.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'caught up' says the letters should be written backwards (in a down clue, letters go up).
'in' is an insertion indicator.
'attack' becomes 'haveat' (to have at someone is to attack them).
'i'+'f'='if'
'if' backwards is 'fi'.
'fi' inserted into 'haveat' is 'HAVE A FIT'.
(Other definitions for have a fit that I've seen before include "flare up" , "Become very angry" .)