Unknown individual in band (4)
I believe the answer is:
zone
'band' is the definition.
(thesaurus)
'unknown individual' is the wordplay.
'unknown' becomes 'z' (algebra).
'individual' becomes 'one'.
'z'+'one'='ZONE'
'in' acts as a link.
(Other definitions for zone that I've seen before include "Bitter, pungent" , "A defined area as for particular purpose" , "Area, division" , "Area with a particular characteristic" , "Region; time area" .)