Mostly happy to host a social event (4)
I believe the answer is:
gala
'event' is the definition.
(I have seen 'Festive event ' mean 'gala' so perhaps 'event' could also mean 'gala')
'mostly happy to host a social' is the wordplay.
'mostly' means to remove the last letter.
'happy' becomes 'gay' (similar in meaning).
'to host a social' becomes 'la' (I can't explain this - if you can you should give a lot more credence to this answer).
'gay' with its final letter removed is 'ga'.
'ga'+'la'='GALA'
Can you help me to learn more?
(Other definitions for gala that I've seen before include "A festive occasion, perhaps of sporting nature" , "field day" , "Festive occasion - variety of apple" , "sporting event" , "Festive event" .)