Leader in athletics by virtue of being light blue (4)
I believe the answer is:
aqua
'light blue' is the definition.
(I've seen this in another clue)
'leader in athletics by virtue of' is the wordplay.
'leader in' says to take the initial letters.
'by' says to put letters next to each other.
'virtue of' becomes 'qua' (I've seen this in other clues).
The first letter of 'athletics' is 'a'.
'a'+'qua'='AQUA'
'being' acts as a link.
(Other definitions for aqua that I've seen before include "Light bluish-green -- water" , "- Regia; - fortis" , "Water (L)" , "Blue tinged with green" , "shade" .)