Doctor on round longs to be amusing (5)
I believe the answer is:
droll
'be amusing' is the definition.
(thesaurus)
'doctor on round longs' is the wordplay.
I cannot really understand how this works, but
'doctor' could be 'dr' ('Dr' can be a synonym of 'doctor') and 'dr' is located in the answer.
'round' could be 'o' (round can mean a round object) and 'o' is present in the answer.
This explanation may well be incorrect...
'to' acts as a link.
Can you help me to learn more?
(Other definitions for droll that I've seen before include "Large amount (of matter)" , "Amusing - jester" , "Humorous or whimsical" , "Drily amusing" , "Mildly amusing" .)