Part of marina valued as relating to warships (5)
I believe the answer is:
naval
'relating to warships' is the definition.
(I've seen this before)
'part of marina valued as' is the wordplay.
'part of' says the answer is hidden in the clue.
'NAVAL' is hidden within 'marina valued as'.
(Other definitions for naval that I've seen before include "Relating to a nation's fleet" , "Maritime" , "Relating to nautical defence force" , "Relates to nautical defence force" , "Relating to sea forces" .)