Young woman entertains boss, heading off weariness (9)
I believe the answer is:
lassitude
'weariness' is the definition.
(thesaurus)
'young woman entertains boss heading off' is the wordplay.
'young woman' becomes 'lassie' (lassie is a kind of young woman).
'entertains' indicates putting letters inside.
'boss heading off' becomes 'tud' (I can't explain this - if you can you should believe this answer much more).
'lassie' enclosing 'tud' is 'LASSITUDE'.
Can you help me to learn more?
(Other definitions for lassitude that I've seen before include "Listlessness, state of apathy" , "Weakness" , "slackness" , "Lack of energy and enthusiasm" , "Weariness" .)