Unhappy daughter has extremely large burden (6)
I believe the answer is:
saddle
'burden' is the definition.
('saddle' can be a synonym of 'burden')
'unhappy daughter has extremely large' is the wordplay.
'unhappy' becomes 'sad' ('sad' can be a synonym of 'unhappy'**).
'daughter' becomes 'd'.
'has' means one lot of letters go next to another.
'extremely' means to remove the middle letters.
'large' with its middle removed is 'le'.
'sad'+'d'+'le'='SADDLE'
(Other definitions for saddle that I've seen before include "Seat for riding" , "Lumber (with); one on horse" , "Butcher's cut from the back of an animal" , "Cut of meat" , "Leathery" .)