Fossil fuel finally found in most of the German state (5)
I believe the answer is:
relic
'fossil' is the definition.
(I've seen this in another clue)
'fuel finally found in most of the german state' is the wordplay.
'finally' says to take the final letters.
'found in' is an insertion indicator.
'most of' means to remove the last letter.
'the german state' becomes 'reich' (I've seen this in another clue).
The final letter of 'fuel' is 'l'.
'reich' with its final letter taken off is 'reic'.
'l' put within 'reic' is 'RELIC'.
(Other definitions for relic that I've seen before include "Ossified survival" , "Holy remnant" , "Object of historic interest" , "antique item" , "Outdated survival" .)