Partridge about to seize unknown antelope (5)
I believe the answer is:
nyala
'antelope' is the definition.
(nyala is a kind of antelope)
'partridge about to seize unknown' is the wordplay.
'partridge' becomes 'Alan' (comedy character Alan Partridge).
'about' is a reversal indicator.
'to seize' is an insertion indicator.
'unknown' becomes 'y' (algebra).
'alan' reversed gives 'nala'.
'nala' going around 'y' is 'NYALA'.
(Other definitions for nyala that I've seen before include "one of a herd" , "beast" , "Large African antelope" , "Spiral-horned antelope" , "South African antelope - Sudanese city" .)