After short time humour a fool (4)
I believe the answer is:
twit
'a fool' is the definition.
(thesaurus)
'after short time humour' is the wordplay.
'after' means one lot of letters go next to another.
'short time' becomes 't' (abbreviation for time).
'humour' becomes 'wit' ('wit' can be a synonym of 'humour').
't'+'wit' is 'TWIT'.
(Other definitions for twit that I've seen before include "Twerp" , "Mild word for a fool" , "Slangily a foolish person" , "Ninny" , "Idiot; tease" .)