Catch tup mounting another animal (6)
I believe the answer is:
marten
'another animal' is the definition.
'marten' can be an answer for 'animal' (thesaurus). I'm unsure of the 'another' bit.
'catch tup mounting' is the wordplay.
'catch' becomes 'net' (netting is a kind of catching).
'tup' becomes 'ram' ('ram' can be a synonym of 'tup').
'mounting' says the letters should be written backwards (going up in a down clue).
'net'+'ram'='netram'
'netram' back-to-front is 'MARTEN'.
(Other definitions for marten that I've seen before include "forest-dweller" , "Animal - rat men (anag)" , "Weasel-like animal with thick soft fur" , "Weasel-like mammal of forests" , "Furry carnivore, maybe of pine woods" .)