Marine soldier trapped in unusual place (7)
I believe the answer is:
pelagic
'marine' is the definition.
(I've seen this before)
'soldier trapped in unusual place' is the wordplay.
'soldier' becomes 'gi' (GI can mean a US soldier).
'trapped in' means one lot of letters goes inside another.
'unusual' is an anagram indicator.
'place' anagrammed gives 'pelac'.
'gi' going into 'pelac' is 'PELAGIC'.
(Other definitions for pelagic that I've seen before include "of the ocean" , "Like an albatross" , "Marine" , "Sea" , "Inhabiting the deep" .)