Chap has one chief (4)
I believe the answer is:
main
'chief' is the definition.
(synonyms)
'chap has one' is the wordplay.
'chap' becomes 'man' (chap can mean a man).
'has' means one lot of letters goes inside another.
'one' becomes 'I' (Roman numeral).
'man' enclosing 'i' is 'MAIN'.
(Other definitions for main that I've seen before include "Most important element" , "Foremost" , "Principal or high seas" , "Most important - high seas?" , "source of gas" .)