Sailor Small accompanies really cool friend (8)
I believe the answer is:
shipmate
'sailor' is the definition.
(I have seen 'Fellow sailor' mean 'shipmate' so perhaps 'sailor' could also mean 'shipmate')
'small accompanies really cool friend' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'accompanies' says to put letters next to each other.
'really cool' becomes 'hip' (hip can mean fashionable).
'friend' becomes 'mate' (mate is a kind of friend).
's'+'hip'+'mate'='SHIPMATE'
(Other definitions for shipmate that I've seen before include "Fellow crewman" , "Fellow seaman" , "Sailor on board, a real pal" , "Friend on board" , "Fellow crew member" .)