Farah's lower in capital (6)
I believe the answer is:
moscow
'capital' is the definition.
(capital of Russia)
'farah's lower' is the wordplay.
'farah's' becomes 'Mo's' (Sir Mo Farah, British Olympian).
'lower' becomes 'cow' (something which lows - makes cow sounds).
'mos'+'cow'='MOSCOW'
'in' is the link.
(Other definitions for moscow that I've seen before include "Russian capital" , "capital city" , "Bolshoi Ballet base" , "Russian city" , "City of 13" .)