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' is an anagram indicator.
'by' says to put letters next to each other.
'sailors' becomes 'RN' (abbreviation for Royal Navy).
'set' is an anagram of 'ste'.
'ste'+'rn'='STERN'
'getting' acts as a link.
(Other definitions for stern that I've seen before include "Demanding < back" , "Severe, demanding" , "Severe; grimly serious" , "Grim, strict" , "Grim; severe" .)