Big coal admits defeat (8)
I believe the answer is:
colossal
'big' is the definition.
(similar in meaning)
'coal admits defeat' is the wordplay.
'admits' is an insertion indicator (to admit is to allow in).
'defeat' becomes 'loss' (I've seen this before).
'coal' placed around 'loss' is 'COLOSSAL'.
(Other definitions for colossal that I've seen before include "Enormous, gigantic" , "Massive" , "Great" , "Stupendous" , "Very big" .)