Nag is a sort of animal (6)
I believe the answer is:
badger
This clue could be a double definition.
'nag' is the first definition.
(I've seen this in another clue)
'sort of animal' is the second definition.
'badger' can be an answer for 'animal' (I've seen this before). I'm unsure of the 'sort of' bit.
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for badger that I've seen before include "protected species" , "who pins an emblem on?" , "quarry, perhaps" , "Nocturnal black and white animal" , "Carnivore - pester" .)