Discharge ambassador, who's in drink (5)
I believe the answer is:
rheum
'discharge' is the definition.
(I've seen this before)
'ambassador who's in drink' is the wordplay.
'ambassador' becomes 'he' (His or Her Excellency).
'who's in' is an insertion indicator.
'drink' becomes 'rum' (rum is a kind of drink).
'he' inserted into 'rum' is 'RHEUM'.
(Other definitions for rheum that I've seen before include "Watery discharge; sounds like space" , "nasal discharge" , "drips from the nose" , "Discharge of water" , "Tears of a cold" .)