Mammal seeing reptile slip off (11)
I believe the answer is:
pipistrelle
'mammal' is the definition.
(pipistrelle is a kind of mammal)
'reptile slip off' is the wordplay.
'off' indicates an anagram.
'reptile'+'slip'='reptileslip'
'reptileslip' is an anagram of 'PIPISTRELLE'.
'seeing' acts as a link.
(Other definitions for pipistrelle that I've seen before include "Type of bat" , "Small variety of bat" .)