Stupidly rich, devoted man joins mother and sons for feast-day (9)
I believe the answer is:
christmas
'day' is the definition.
(Christmas is an example)
'stupidly rich devoted man joins mother and sons' is the wordplay.
'stupidly rich devoted man joins' becomes 'christ' (I can't explain this - if you can you should believe this answer much more).
'mother' becomes 'ma' (ma can mean mother).
'and' says to put letters next to each other.
'sons' becomes 's'.
'christ'+'ma'+'s'='CHRISTMAS'
'for feast' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for christmas that I've seen before include "Not in general use" , "Winter festival" , "Holy festival" , "- box; - tree" , "see 11" .)