Arch nemesis' leader engaged in plot (4)
I believe the answer is:
bend
'arch' is the definition.
(arch can mean to bend)
'nemesis' leader engaged in plot' is the wordplay.
'leader' suggests taking the first letters.
'engaged in' indicates putting letters inside.
'plot' becomes 'bed' (a plot of land).
The initial letter of 'nemesis' is 'n'.
'n' inserted inside 'bed' is 'BEND'.
(Other definitions for bend that I've seen before include "Influence, bow" , "Knot; heraldic stripe" , "Crouch" , "Turn - buckle" , "Stoop; (heraldic) stripe" .)