Help put up lightweight sketch (7)
I believe the answer is:
diagram
'sketch' is the definition.
(I've seen this before)
'help put up lightweight' is the wordplay.
'help' becomes 'aid' (synonyms).
'put up' is a reversal indicator.
'lightweight' becomes 'gram'.
'aid' back-to-front is 'dia'.
'dia'+'gram'='DIAGRAM'
(Other definitions for diagram that I've seen before include "scheme" , "Drawing to show how things work" , "Outline drawing showing parts or workings" , "Drawing or blueprint" , "A grid, Ma? (Anag)" .)