Unfavourable criticism is always a blow (5)
I believe the answer is:
knock
'a blow' is the definition.
(knock is a kind of blow)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for knock that I've seen before include "Tap on door" , "Bash" , "Fail to hit" , "Bang" , "Unfairly criticise" .)