Led off bearing unhappy burden (6)
I believe the answer is:
saddle
'burden' is the definition.
('saddle' can be a synonym of 'burden')
'led off bearing unhappy' is the wordplay.
'off' indicates anagramming the letters.
'bearing' means one lot of letters goes inside another.
'unhappy' becomes 'sad' ('sad' can be a synonym of 'unhappy'**).
'led' is an anagram of 'dle'.
'dle' placed around 'sad' is 'SADDLE'.
(Other definitions for saddle that I've seen before include "society's bad" , "Impose a task on -- assign responsibility for" , "Encumber" , "A horseman's seat" , "Burden; seat" .)