The doctor's a bit of a fool to enthuse unduly (5)
I believe the answer is:
drool
'a fool to enthuse unduly' is the definition.
I can't tell whether this definition defines the answer.
'the doctor's a bit' is the wordplay.
'the doctor' becomes 'Dr' (abbreviation).
'a bit' becomes 'ool' (I can't justify this - if you can you should give a lot more credence to this answer).
'dr'+'ool'='DROOL'
'of' is the link.
Can you help me to learn more?
(Other definitions for drool that I've seen before include "Dogs may" , "dribble" , "Spill saliva" , "Salivate" , "Show excitement" .)
