Limb inserted into upset male animal (6)
I believe the answer is:
marmot
'animal' is the definition.
(marmot is a kind of animal)
'limb inserted into upset male' is the wordplay.
'limb' becomes 'arm' (arm is a kind of limb).
'inserted into' indicates putting letters inside.
'upset' is a reversal indicator.
'male' becomes 'tom' (I've seen this before).
'tom' in reverse letter order is 'mot'.
'arm' inserted within 'mot' is 'MARMOT'.
(Other definitions for marmot that I've seen before include "Creature" , "Furry burrowing rodent" , "Stout, burrowing rodent" , "wild rodent" , "Coarse-furred burrowing rodent" .)