Left one kind of mammal means of escape (8)
I believe the answer is:
lifeline
'means of escape' is the definition.
(I've seen this before)
'left one kind of mammal' is the wordplay.
'left' becomes 'l' (common abbreviation).
'one' becomes 'i' (Roman numeral).
'kind of mammal' becomes 'feline' ().
'l'+'i'+'feline'='LIFELINE'
(Other definitions for lifeline that I've seen before include "in the palm of one's hand" , "Essential means of support" , "Means of escape" , "rescue operation?" , "vital means of communication" .)