Butt of lowbrow gag — poor fellow (6)
I believe the answer is:
wretch
'poor fellow' is the definition.
(poor or unfortunate person)
'butt of lowbrow gag' is the wordplay.
'butt of' says to take the final letters (the end or 'butt' of the word).
'gag' becomes 'retch' (I've seen this in other clues).
The last letter of 'lowbrow' is 'w'.
'w'+'retch'='WRETCH'
(Other definitions for wretch that I've seen before include "Swine" , "Poor miserable creature" , "Person in pitiful circumstances" , "Unfortunate, miserable person" , "Scoundrel" .)