Old lady heartlessly trampled ancient creature (8)
I believe the answer is:
mastodon
'ancient creature' is the definition.
(extinct creature)
'old lady heartlessly trampled' is the wordplay.
'old lady' becomes 'ma' (both can informally mean mother).
'heartlessly' suggests removing the centre.
'trampled' becomes 'stood on'.
'stoodon' with its middle removed is 'stodon'.
'ma'+'stodon'='MASTODON'
(Other definitions for mastodon that I've seen before include "giant from the past" , "Extinct beast" , "Extinct mammal - man stood (anag)" , "Extinct elephant-like mammal" , "old jumbo" .)