They know how to treat animals (4)
I believe the answer is:
vets
'they know how' is the definition.
The definition suggests an adverb but the answer is not.
'treat animals' is the wordplay.
'treat' becomes 'vet' (vetting is a kind of treating).
'animals' becomes 's' (I can't justify this - if you can you should give a lot more credence to this answer).
'vet'+'s'='VETS'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for vets that I've seen before include "Inspects (animals?)" , "Critically examines" , "Animal surgeons" , "Screens for the job perhaps" , "Examines the animal doctors carefully?" .)