Harmful parasite, mostly found on the bovine exterior (5)
I believe the answer is:
toxic
'harmful' is the definition.
(associated in meaning)
'found on the bovine exterior' is the wordplay.
'found on' says to put letters next to each other.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'bovine' becomes 'ox' (ox is a kind of bovine).
'exterior' becomes 'ic' (I can't justify this - if you can you should believe this answer much more).
't' next to 'ox' is 'tox'.
'tox'+'ic'='TOXIC'
'parasite mostly' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for toxic that I've seen before include "Poisonous, noxious" , "Harmful - cox it (anag)" , "word describing bad things" , "Deadly - venomous" .)