Spoil good welfare, having reversed plan (7)
I believe the answer is:
diagram
'plan' is the definition.
(I've seen this before)
'spoil good welfare having reversed' is the wordplay.
'spoil' becomes 'mar' (I've seen this before).
'good' becomes 'g' (abbreviation).
'welfare' becomes 'aid' (I've seen this before).
'having reversed' is a reversal indicator.
'mar'+'g'+'aid'='margaid'
'margaid' backwards is 'DIAGRAM'.
(Other definitions for diagram that I've seen before include "scheme" , "Schematic representation" , "Explanatory illustration" , "Outline drawing showing parts or workings" , "Simplified drawing" .)