Animal doctor tense after sheep rejected (6)
I believe the answer is:
marmot
'animal' is the definition.
(marmot is a kind of animal)
'doctor tense after sheep rejected' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'tense' becomes 't' (abbreviation used in many dictionaries).
'after' means one lot of letters go next to another.
'sheep' becomes 'ram' (male sheep).
'rejected' shows that the letters should be reversed in order.
'ram' back-to-front is 'mar'.
'mo'+'t'='mot'
'mot' after 'mar' is 'MARMOT'.
(Other definitions for marmot that I've seen before include "Coarse-furred burrowing rodent" , "Creature" , "Small burrowing animal of the squirrel family" , "Furry burrowing rodent" , "Stout, burrowing rodent" .)