Keen son fills post? It's for the better (10)
I believe the answer is:
sweepstake
'for the better' is the definition.
I can't judge whether this definition defines the answer.
'keen son fills post?' is the wordplay.
'keen' becomes 'weep' (keen can mean to cry or lament).
'son' becomes 's' (genealogical abbreviation for son).
'fills' means one lot of letters goes inside another.
'post?' becomes 'stake' (synonyms).
'weep'+'s'='weeps'
'weeps' placed inside 'stake' is 'SWEEPSTAKE'.
'it's' is the link.
Can you help me to learn more?
(Other definitions for sweepstake that I've seen before include "I'd bet on it" , "action on race" , "Lottery" , "lots here" , "prize" .)