Realise American has boarded ship (4)
I believe the answer is:
suss
'realise' is the definition.
(I've seen this before)
'american has boarded ship' is the wordplay.
'american' becomes 'us' (United States).
'has boarded' indicates putting letters inside (inserted letters come on board).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'us' inserted within 'ss' is 'SUSS'.
(Other definitions for suss that I've seen before include "Work (out) (slang)" , "Discover (slang)" , "sharpness of mind" , "Grasp, realise (slang)" , "Behaviour interesting police" .)