Rescue five in rough sea (4)
I believe the answer is:
save
'rescue' is the definition.
(I know that save is a more specific form of the action rescue)
'five in rough sea' is the wordplay.
'five' becomes 'V' (Roman numeral).
'in' means one lot of letters goes inside another.
'rough' indicates an anagram (a rough or broken ordering of the letters).
'sea' anagrammed gives 'sae'.
'v' put within 'sae' is 'SAVE'.
(Other definitions for save that I've seen before include "Preserve; except" , "Rescue for a rainy day?" , "Rescue; preserve" , "Be economical" , "Hang on to" .)