Bitter, bitter having leaked runs (4)
I believe the answer is:
acid
'bitter' is the definition.
('acid' can be a synonym of 'bitter')
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for acid that I've seen before include "Hallucinogenic drug" , "Morose" , "Substance with pH of less than 7" , "Bitter; biting" , "Biting - tart" .)