Runner, one between two runs (5)
I believe the answer is:
racer
'runner' is the definition.
(thesaurus)
'one between two runs' is the wordplay.
'one' becomes 'ace' ('ace' can be a synonym of 'one'**).
'between' means one lot of letters goes inside another.
'two' indicates the doubling of some letters.
'runs' becomes 'r' (cricket abbreviation).
'r' duplicated is 'rr'.
'ace' placed into 'rr' is 'RACER'.
(Other definitions for racer that I've seen before include "someone driving very fast?" , "Perhaps 16 [SCRAMBLER]" , "Speedway competitor, perhaps" , "Bike, perhaps" , "American snake" .)