Jacket mostly in flames (5)
I believe the answer is:
blaze
'jacket' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'mostly in flames' is the wordplay.
'mostly' means to remove the first letter.
'in flames' becomes 'ablaze' (I've seen this in another clue).
'ablaze' with its initial letter taken off is 'BLAZE'.
Can you help me to learn more?
(Other definitions for blaze that I've seen before include "Burn brightly" , "Pioneer" , "Vivid display" , "Strong fire" , "Fiercely burning fire" .)