Top British runner keeps left (6)
I believe the answer is:
blouse
'top' is the definition.
(blouse is a kind of top)
'british runner keeps left' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'runner' becomes 'ouse' (Yorkshire river**).
'keeps' means one lot of letters goes inside another.
'left' becomes 'l' (common abbreviation).
'b'+'ouse'='bouse'
'bouse' placed around 'l' is 'BLOUSE'.
(Other definitions for blouse that I've seen before include "Garment like a shirt" , "Top garment - so blue (anag)" , "Female garment" , "Within a building" , "Woman's buttoned top" .)