Horse disturbs last big cat (8)
I believe the answer is:
stallion
'horse' is the definition.
(I've seen this before)
'disturbs last big cat' is the wordplay.
'disturbs' is an anagram indicator (I've seen 'disturbed' mean this).
'big cat' becomes 'lion' (lion is a kind of big cat).
'last' is an anagram of 'stal'.
'stal'+'lion'='STALLION'
(Other definitions for stallion that I've seen before include "Male steed" , "Male horse kept for breeding" , "Mare's mate" , "Male horse lost in atolls" , "Uncastrated adult male horse" .)