Whip for poor act (3)
I believe the answer is:
cat
'whip' is the definition.
(cat o' nine tails, cat for short is a type of whip)
'poor act' is the wordplay.
'poor' is an anagram indicator.
'act' anagrammed gives 'CAT'.
'for' is the link.
(Other definitions for cat that I've seen before include "Tiger, for instance" , "An old whip with nine tails" , "Domestic animal" , "Lash < jazz fan" , "Feline mammel" .)