Think highly of large mammal (5)
I believe the answer is:
ratel
'mammal' is the definition.
(ratel is a kind of mammal)
'think highly of large' is the wordplay.
'think highly of' becomes 'rate' (I've seen this before).
'large' becomes 'L' (eg in clothes sizes).
'rate'+'l'='RATEL'
(Other definitions for ratel that I've seen before include "One eats honey" , "creature with sweet tooth" , "animal like a badger" , "Badger-like creature" , "Mammal's" .)