Slump preceding a long tale (4)
I believe the answer is:
saga
'long tale' is the definition.
(I've seen this before)
'slump preceding a' is the wordplay.
'slump' becomes 'sag' (I've seen this before).
'preceding' says to put letters next to each other.
'sag'+'a'='SAGA'
(Other definitions for saga that I've seen before include "Long heroic, family, story" , "Long detailed story" , "Flag associated with a" , "Epic - legend" , "Norse epic" .)