Marine in actual trouble (8)
I believe the answer is:
nautical
'marine' is the definition.
(synonyms)
'in actual trouble' is the wordplay.
'trouble' indicates anagramming the letters.
'in'+'actual'='inactual'
'inactual' with letters rearranged gives 'NAUTICAL'.
(Other definitions for nautical that I've seen before include "Of ships and navigation" , "like a sailor" , "Relating to ships and sailing" , "A lunatic about sailors or seamanship" , "concerning ratings, etc?" .)