Sneer at a good fellow (5)
I believe the answer is:
scout
'sneer at' is the definition.
(thesaurus)
'a good fellow' is the wordplay.
I cannot really see how this works, but
'fellow' could be 'co' (I've seen this in other clues) and 'co' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for scout that I've seen before include "One searches for and recruits talented people" , "Talent seeker - boy on a jamboree" , "Reconnoitre" , "Soldier on a reconnaissance mission" , "Soldier on a recce mission" .)