Cat is a disciplinarian (4)
I believe the answer is:
whip
This clue is a double definition.
'cat' is the first definition.
(cat o' nine tails or cat is a type of whip)
'disciplinarian' is the second definition.
(I have seen 'Parliamentary disciplinarian' mean 'whip' so perhaps 'disciplinarian' could also mean 'whip')
'is a' acts as a link.
(Other definitions for whip that I've seen before include "Scourge; steal" , "MP enforcing discipline" , "Party disciplinarian" , "Party official; punish" , "Person who enforces discipline in a political party" .)