Around capital, unknown peak (6)
I believe the answer is:
climax
'peak' is the definition.
(I've seen this before)
'around capital unknown' is the wordplay.
'around' becomes 'c' (abbreviation for 'circa').
'capital' becomes 'lima' (capital of Peru).
'unknown' becomes 'x' (algebra).
'c'+'lima'+'x'='CLIMAX'
(Other definitions for climax that I've seen before include "Peak of a performance" , "Most exciting point" , "The decisive moment, in a novel or play say" , "Most intense moment" , "exciting finale" .)