Capital unknown before taking lead (7)
I believe the answer is:
yerevan
'capital' is the definition.
(capital of Armenia)
'unknown before taking lead' is the wordplay.
'unknown' becomes 'y' (algebra).
'before' becomes 'ere' (poetic term).
'taking' says to put letters next to each other.
'lead' becomes 'van' (van is the leading part of a movement or group).
'y'+'ere'+'van'='YEREVAN'
(Other definitions for yerevan that I've seen before include "Capital of Armenia" , "east European city" , "ancient capital" .)