Plan demands help to lift weight (7)
I believe the answer is:
diagram
'plan' is the definition.
(I've seen this in another clue)
'help to lift weight' is the wordplay.
'help' becomes 'aid' (synonyms).
'to lift' is a reversal indicator (in a down clue, letters lift up).
'weight' becomes 'gram' (unit of weight).
'aid' reversed gives 'dia'.
'dia'+'gram'='DIAGRAM'
'demands' acts as a link.
(Other definitions for diagram that I've seen before include "Sketch - plan" , "Chart" , "Explanatory drawing or plan" , "Explatory drawing" , "Explanatory figure" .)