Burden with second muddle (6)
I believe the answer is:
saddle
'burden' is the definition.
('saddle' can be a synonym of 'burden')
'with second muddle' is the wordplay.
'with' says to put letters next to each other.
'second' becomes 's' (common abbreviation**).
'muddle' becomes 'addle' ('addle' can be a synonym of 'muddle').
's' put next to 'addle' is 'SADDLE'.
(Other definitions for saddle that I've seen before include "A horseman's seat" , "Equestrian seat" , "Burden: pass" , "Seat for a rider, horse or bike" , "land" .)