Lengthy account of a gas explosion (4)
I believe the answer is:
saga
'lengthy account' is the definition.
(a saga is a lengthy narrative)
'a gas explosion' is the wordplay.
'explosion' indicates anagramming the letters.
'a'+'gas'='agas'
'agas' anagrammed gives 'SAGA'.
'of' acts as a link.
(Other definitions for saga that I've seen before include "Long narrative" , "Icelandic or Norse story" , "Heroic narrative" , "Tale of heroes" , "Long and involved story" .)