Condescend to take on board son's plan (6)
I believe the answer is:
design
'plan' is the definition.
(I know that plan can be written as design)
'condescend to take on board son's' is the wordplay.
'condescend' becomes 'deign' (synonyms).
'to take on board' indicates putting letters inside.
'deign' enclosing 's' is 'DESIGN'.
(Other definitions for design that I've seen before include "Plan for specific purpose" , "Conceive" , "Develop or prepare a plan" , "Singed (anag.)" , "Provide blueprint" .)