Secure top of foresail in rough sea (4)
I believe the answer is:
safe
'secure' is the definition.
(I know that secure can be written as safe)
'top of foresail in rough sea' is the wordplay.
'top of' suggests taking the first letters.
'in' is an insertion indicator.
'rough' indicates anagramming the letters (a rough or broken ordering of the letters).
The initial letter of 'foresail' is 'f'.
'sea' with letters rearranged gives 'sae'.
'f' placed within 'sae' is 'SAFE'.
(Other definitions for safe that I've seen before include "Cash-box" , "Uninjured, secure" , "Strongbox; secure" , "Place for keeping valuables" , "Secure and protected" .)