Fail to take on board marine's complaint (7)
I believe the answer is:
disease
'complaint' is the definition.
(I've seen this before)
'fail to take on board marine's' is the wordplay.
'fail' becomes 'die' (synonyms).
'to take on board' means one lot of letters goes inside another.
'marine's' becomes 'sea's' (both can mean 'of the sea').
'die' going around 'seas' is 'DISEASE'.
(Other definitions for disease that I've seen before include "22 [MALADY]" , "Bodily disorder" , "Impairment of health" , "scrapie for example" , "scurvy?" .)