Say again about solid fuel (6)
I believe the answer is:
repeat
'say again' is the definition.
(I've seen this before)
'about solid fuel' is the wordplay.
'about' becomes 're' (regarding).
'solid fuel' becomes 'peat' (I've seen this in another clue).
're'+'peat'='REPEAT'
(Other definitions for repeat that I've seen before include "Quote from memory" , "Second showing" , "Parrot" , "Divulge" , "Go over" .)