Athlete swallows one more fluid (7)
I believe the answer is:
runnier
'more fluid' is the definition.
(runny can mean fluid or liquid)
'athlete swallows one' is the wordplay.
'athlete' becomes 'runner' (type of athlete).
'swallows' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'runner' going around 'i' is 'RUNNIER'.
(Other definitions for runnier that I've seen before include "More liquid" , "becomes more fluid ..." .)