Male animal pack upset animal (6)
I believe the answer is:
marmot
'animal' is the definition.
(marmot is a kind of animal)
'male animal pack upset' is the wordplay.
'male animal' becomes 'tom' (male cat).
'pack' becomes 'ram' (synonyms).
'upset' shows that the letters should be reversed in order.
'tom'+'ram'='tomram'
'tomram' in reverse letter order is 'MARMOT'.
(Other definitions for marmot that I've seen before include "Coarse-furred burrowing rodent" , "wild rodent" , "Large burrowing rodent" , "Furry burrowing rodent" , "Stout, burrowing rodent" .)