Month taken over Fitzgerald story of no great length (7)
I believe the answer is:
novella
'story of no great length' is the definition.
(novellas are short literary works)
'month taken over fitzgerald' is the wordplay.
'month' becomes 'Nov' (abbreviation for November).
'taken over' says to put letters next to each other (in a down clue, some letters go above others).
'fitzgerald' becomes 'Ella' (musician Ella Fitzgerald).
'nov'+'ella'='NOVELLA'
(Other definitions for novella that I've seen before include "Slim work of fiction" , "Slim volume of fiction" , "Small book" , "piece of fiction" , "Short work of fiction" .)