Passed drink round, one happy day (5)
I believe the answer is:
given
'passed' is the definition.
The answer and definition can be both related to change as well as being past participle verbs.
Perhaps there's a link between them I don't understand?
'drink round one happy day' is the wordplay.
'drink' becomes 'gin' (gin is an alcoholic drink).
'round' means one lot of letters goes inside another.
'one happy day' becomes 've' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'gin' enclosing 've' is 'GIVEN'.
Can you help me to learn more?
(Other definitions for given that I've seen before include "Taken as read" , "Specified, stated" , "Lent?" , "Donated" , "supplied" .)