Jackass departs on cue (6)
I believe the answer is:
donkey
'jackass' is the definition.
(thesaurus)
'departs on cue' is the wordplay.
'departs' becomes 'd' (common abbreviation e.g. train timetables).
'cue' becomes 'key'.
'd'+'on'+'key'='DONKEY'
(Other definitions for donkey that I've seen before include "Beasts of burden" , "Garment associated with building workers" , "Braying animal" , "dope" , "Dunce" .)