Long tale of gas rising over central Spain (4)
I believe the answer is:
saga
'long tale' is the definition.
(I've seen this before)
'gas rising over central spain' is the wordplay.
'rising' says the letters should be written in reverse (going up in a down clue).
'over' says to put letters next to each other.
'central' indicates the central letters.
The middle letter of 'spain' is 'a'.
'gas' back-to-front is 'sag'.
'sag'+'a'='SAGA'
'of' acts as a link.
(Other definitions for saga that I've seen before include "long series of events" , "Long involved tale" , "Epic tale" , "Lengthy tale" , "Heroic narrative" .)