Stop, thanks to my first fowl (6)
I believe the answer is:
bantam
'fowl' is the definition.
(bantam is a kind of fowl)
'stop thanks to my first' is the wordplay.
'stop' becomes 'ban' (I've seen this in another clue).
'thanks' becomes 'ta' (UK informal version of 'thank you').
'to my first' becomes 'm' (1st letter of 'my').
'ban'+'ta'+'m'='BANTAM'
(Other definitions for bantam that I've seen before include "aggressive little sort" , "Small chicken - small soldier" , "Dwarf variety of domestic fowl" , "Small breed of chicken" , "Breed of domestic fowl" .)