Remove secondary plot (8)
I believe the answer is:
subtract
'remove' is the definition.
(I've seen this before)
'secondary plot' is the wordplay.
'secondary' becomes 'sub' (prefix meaning secondary or deputy).
'plot' becomes 'tract' (land).
'sub'+'tract'='SUBTRACT'
(Other definitions for subtract that I've seen before include "Deduct" , "dock" , "Take away (a number)" , "Take an amount away" , "Remove" .)