Knock down a bloke (5)
I believe the answer is:
fella
'bloke' is the definition.
(non-standard spelling of 'fellow')
'knock down a' is the wordplay.
'knock down' becomes 'fell' (I've seen this before).
'fell'+'a'='FELLA'
(Other definitions for fella that I've seen before include "Bloke" , "Chap" , "Guy" .)