Popular mix of fun and zeal is something very infectious (9)
I believe the answer is:
influenza
'something very infectious' is the definition.
I can't tell whether this definition defines the answer.
'popular mix of fun and zeal' is the wordplay.
'popular' becomes 'in'.
'mix of' indicates an anagram.
'and' is a charade indicator (letters next to each other) (I've seen this in other clues).
'fun' put after 'zeal' is 'zealfun'.
'zealfun' with letters rearranged gives 'fluenza'.
'in'+'fluenza'='INFLUENZA'
'is' is the link.
Can you help me to learn more?
(Other definitions for influenza that I've seen before include "Highly infectious viral disease" , "Nasty viral disease, usually of winter" , "Winter virus disease, even birds get it" , "Respiratory complaint" , "Common infection" .)