Be condescending about son's plan (6)
I believe the answer is:
design
'plan' is the definition.
(I know that plan can be written as design)
'be condescending about son's' is the wordplay.
'be condescending' becomes 'deign' (to deign is to stoop to something beneath oneself).
'about' is an insertion indicator.
'deign' placed around 's' is 'DESIGN'.
(Other definitions for design that I've seen before include "Aim -- decorative pattern" , "Devise, dreate, make out a plan" , "Preliminary sketch or plan" , "Draw a plan or scheme" , "Purpose, pattern" .)