Argue with English reserve (5)
I believe the answer is:
spare
'reserve' is the definition.
(synonyms)
'argue with english' is the wordplay.
'argue' becomes 'spar' (sparring is a kind of arguing).
'with' means one lot of letters go next to another.
'english' becomes 'e' (abbreviation).
'spar'+'e'='SPARE'
(Other definitions for spare that I've seen before include "show mercy" , "Extra item just in case" , "With no excess fat" , "Extra; lean" , "Thin; elegantly simple" .)