Perhaps John Lennon caught fan with legal high (5,3)
I believe the answer is:
betel nut
'perhaps john lennon caught' is the definition.
I can't judge whether this defines the answer.
'fan with legal high' is the wordplay.
'fan' becomes 'nut' (both can mean an enthusiast).
'with' is a charade indicator (letters next to each other).
'legal high' becomes 'betel' (I am not sure about this - if you are sure you should believe this answer much more).
'nut' after 'betel' is 'BETEL-NUT'.
Can you help me to learn more?
(Other definitions for betel nut that I've seen before include "stimulant" , "Seed of a palm chewed (with its leaves) as a narcotic" , "supplies a narcotic" , "that's found on palm" .)