Son to accept post (5)
I believe the answer is:
stake
'post' is the definition.
('stake' can be a synonym of 'post')
'son to accept' is the wordplay.
'son' becomes 's'.
'to accept' becomes 'take' (taking is a kind of accepting).
's'+'take'='STAKE'
(Other definitions for stake that I've seen before include "Amount bet" , "A wager or pointed pole" , "Money gambled" , "Wooden post; ante" , "Financial interest" .)