Working's funny? Not in the morning (5)
I believe the answer is:
using
'working's' is the definition.
(thesaurus)
'funny? not in the morning' is the wordplay.
'funny?' becomes 'amusing' (I've seen this before).
'not' indicates named letters should be taken away.
'in the morning' becomes 'am'.
'amusing' with 'am' taken away is 'USING'.
(Other definitions for using that I've seen before include "Exploiting for one's own advantage" , "Exercising" , "addiction" , "taking drugs" , "Employing" .)