Condescend to include son in plan (6)
I believe the answer is:
design
'plan' is the definition.
(design is a kind of plan)
'condescend to include son' is the wordplay.
'condescend' becomes 'deign' ('deign' can be a synonym of 'condescend').
'to include' indicates putting letters inside.
'son' becomes 's'.
'deign' enclosing 's' is 'DESIGN'.
'in' acts as a link.
(Other definitions for design that I've seen before include "Singed (anag.)" , "Aim -- decorative pattern" , "project" , "Devise, dreate, make out a plan" , "Plan, blueprint" .)