Mammal climbing Sicilian mountain ate rat's head (8)
I believe the answer is:
anteater
'mammal' is the definition.
(I know that anteater is a type of mammal)
'climbing sicilian mountain ate rat's head' is the wordplay.
'climbing' is a reversal indicator (in a down clue, letters climb up).
'sicilian mountain' becomes 'Etna' (Mount Etna).
'head' suggests taking the first letters.
The first letter of 'rats' is 'r'.
'etna' written backwards gives 'ante'.
'ante'+'ate'+'r'='ANTEATER'
(Other definitions for anteater that I've seen before include "One's looking for workers" , "Long-tongued mammal" , "individual living off workers" , "Long-snouted insectivore" , "soldiers killed by this?" .)