Plan to help the over weight! (7)
I believe the answer is:
diagram
'plan' is the definition.
(I've seen this before)
'help the over weight' is the wordplay.
'help' becomes 'aid' ('aid' can be a synonym of 'help').
'the over' is a reversal indicator.
'weight' becomes 'gram' (I have seen 'Metric weight ' mean 'gram' so perhaps 'weight' could also mean 'gram'**).
'aid' reversed gives 'dia'.
'dia'+'gram'='DIAGRAM'
'to' acts as a link.
(Other definitions for diagram that I've seen before include "Explanatory illustration" , "Drawing or blueprint" , "picture" , "Drawing to show how things work" , "Outline drawing showing parts or workings" .)