Seaman, fifty, seated outside (4)
I believe the answer is:
salt
'seaman' is the definition.
(synonyms)
'fifty seated outside' is the wordplay.
'fifty' becomes 'l' (Roman numeral).
'seated' becomes 'sat' ('sit' can be a synonym of 'seat').
'outside' indicates putting letters inside (some letters go outside others).
'l' put inside 'sat' is 'SALT'.
(Other definitions for salt that I've seen before include "Brackish" , "White crystalline substance" , "Condiment - old sailor" , "Mariner; sodium chloride" , "dry wit" .)