A small island force? I doubt that (2,2)
I believe the answer is:
as if
'i doubt that' is the definition.
('as if' may be said to expression disbelief)
'a small island force?' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'island' becomes 'i' (geographical abbreviation).
'force?' becomes 'F' (symbol used in Physics e.g. f=ma).
'a'+'s'+'i'+'f'='AS IF'
(Other definitions for as if that I've seen before include "Expression of incredulity" , "You're joking!" , "I very much doubt it!" , "a voice of doubt" , "highly unlikely" .)