It's grim, getting set about by sailors! (5)
I believe the answer is:
stern
'it's grim' is the definition.
(synonyms)
'set about by sailors' is the wordplay.
'about' indicates an anagram.
'by' is a charade indicator (letters next to each other).
'sailors' becomes 'RN' (abbreviation for Royal Navy).
'set' is an anagram of 'ste'.
'ste'+'rn'='STERN'
'getting' is the link.
(Other definitions for stern that I've seen before include "Demanding < back" , "Severe, demanding" , "Severe; grimly serious" , "Grim, strict" , "Grim; severe" .)