I chuck in worn-out fossil (9)
I believe the answer is:
trilobite
'fossil' is the definition.
(extinct creatures represented in the fossil record)
'i chuck in worn-out' is the wordplay.
'chuck' becomes 'lob' (I've seen this before).
'in' is an insertion indicator.
'worn out' becomes 'trite' (I've seen this before).
'i'+'lob'='ilob'
'ilob' put inside 'trite' is 'TRILOBITE'.
(Other definitions for trilobite that I've seen before include "find on Lyme Regis beach?" , "old crawler" , "Common marine fossil" , "Extinct arthropod" , "one in rock" .)