Discovered "in it" with donkey (6)
I believe the answer is:
nitwit
'donkey' is the definition.
(thesaurus)
'discovered in it with' is the wordplay.
'discovered' says the answer is hidden in the clue (remove the letters covering the answer in the clue).
'NITWIT' can be found hidden inside 'in it with'.
(Other definitions for nitwit that I've seen before include "Louse egg with humour - silly fellow" , "Birdbrain" , "Plonker" , "Informally, a foolish person, featherbrain" , "Noodle, numskull" .)