Long story about single antelope (5)
I believe the answer is:
saiga
'antelope' is the definition.
(saiga is a kind of antelope)
'long story about single' is the wordplay.
'long story' becomes 'saga' (a saga is a long and complicated story).
'about' is an insertion indicator.
'single' becomes 'I' (I as in one).
'saga' placed around 'i' is 'SAIGA'.
(Other definitions for saiga that I've seen before include "fat-nosed animal?" , "Asian antelope" , "type of antelope" .)