Feast Attlee shortly set about with a son (9)
I believe the answer is:
candlemas
'feast' is the definition.
'attlee shortly set about with a son' is the wordplay.
'attlee shortly' becomes 'clem' (Clement Attlee).
'set about' means one lot of letters goes inside another.
'with' becomes 'and' (I've seen this before**).
'son' becomes 's'.
'clem' enclosing 'and' is 'candlem'.
'candlem'+'a'+'s'='CANDLEMAS'
(Other definitions for candlemas that I've seen before include "church festival" , "feast day" , "Feast of the Purification of the Virgin Mary" , "February Christian festival" .)