A source of mint ran out (9)
I believe the answer is:
informant
'a source' is the definition.
(I know that source can be written as informant)
'of mint ran out' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'of'+'mint'+'ran'='ofmintran'
'ofmintran' is an anagram of 'INFORMANT'.
(Other definitions for informant that I've seen before include "source of intelligence" , "gives us the gen" , "Traitor" , "Snitch" , "Rat" .)