Enthuse as Doctor John returns (5)
I believe the answer is:
drool
'enthuse' is the definition.
Although both the answer and definition are verbs in their base form, I don't see how one could define the other.
'doctor john returns' is the wordplay.
'doctor' becomes 'Dr' (abbreviation).
'john' becomes 'loo' (loo is a kind of john).
'returns' is a reversal indicator.
'loo' backwards is 'ool'.
'dr'+'ool'='DROOL'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for drool that I've seen before include "Spill saliva" , "Show excitement" , "Dogs may" , "Slaver - drivel" , "Slobber" .)