Unknown period needed to capture black animal (5)
I believe the answer is:
zebra
'animal' is the definition.
(thesaurus)
'unknown period needed to capture black' is the wordplay.
'unknown' becomes 'z' (algebra).
'period' becomes 'era' (era is a kind of period**).
'needed' means one lot of letters go next to another.
'to capture' means one lot of letters goes inside another.
'black' becomes 'b' (as in pencil classifications).
'z'+'era'='zera'
'zera' going around 'b' is 'ZEBRA'.
(Other definitions for zebra that I've seen before include "that's stripy" , "African species" , "Stripy member of horse family" , "Animal like striped pony" , "Kind of crossing?" .)