Confirmed spineless creature lacks courage and fibre essentially (10)
I believe the answer is:
inveterate
'confirmed' is the definition.
(thesaurus)
'spineless creature lacks courage and fibre essentially' is the wordplay.
I cannot really understand how this works, but
'creature' could be 'rat' (rat is a kind of creature) and 'rat' is located in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is found within the answer.
This explanation may well be incorrect...
(Other definitions for inveterate that I've seen before include "Firmly established by usage or custom" , "Confirmed in (bad) habit" , "long-standing" , "Long-established" , "Hardened" .)