Say again concerning solid fuel (6)
I believe the answer is:
repeat
'say again' is the definition.
(I've seen this in another clue)
'concerning solid fuel' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'solid fuel' becomes 'peat' (I've seen this in another clue).
're'+'peat'='REPEAT'
(Other definitions for repeat that I've seen before include "Parrot" , "Further airing" , "Second showing" , "old movie" , "Say or perform again" .)