Warriors' one American drink when heading west (7)
I believe the answer is:
samurai
'warriors'' is the definition.
(I've seen this before)
'one american drink when heading west' is the wordplay.
'one' becomes 'I' (Roman numeral).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'drink' becomes 'rum' (rum is a kind of drink).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'heading west' shows that the letters should be reversed in order.
'i'+'a'+'rum'+'as'='iarumas'
'iarumas' written backwards gives 'SAMURAI'.
(Other definitions for samurai that I've seen before include "Military figures" , "Traditional warriors of Japan" , "Caste of Japanese warriors" , "Historical Japanese warrior" , "military man" .)