Old boy repulsed by a snake (3)
I believe the answer is:
boa
'snake' is the definition.
(boa is a kind of snake)
'old boy repulsed by a' is the wordplay.
'old boy' becomes 'ob'.
'repulsed' says the letters should be written in reverse.
'by' says to put letters next to each other.
'ob' reversed gives 'bo'.
'bo'+'a'='BOA'
(Other definitions for boa that I've seen before include "One covered in scales" , "Feathered neckwear - snake" , "feathers around the neck?" , "... cold-blooded creature" , "Large snake provides a neck decoration" .)