Peak in cold capital city, unknown (6)
I believe the answer is:
climax
'peak' is the definition.
(I've seen this before)
'cold capital city unknown' is the wordplay.
'cold' becomes 'c' (eg on taps).
'capital city' becomes 'Lima' (capital of Peru).
'unknown' becomes 'x' (algebra).
'c'+'lima'+'x'='CLIMAX'
'in' acts as a link.
(Other definitions for climax that I've seen before include "Turning point or culmination" , "exciting finale" , "Exciting conclusion" , "Turning point, moment of greatest intensity" , "Culminating moment" .)