A burden is over (6)
I believe the answer is:
across
'over' is the definition.
(synonyms)
'a burden' is the wordplay.
'burden' becomes 'cross' (someone's particular burden is their cross).
'a'+'cross'='ACROSS'
'is' is the link.
(Other definitions for across that I've seen before include "Transversely" , "To the opposite side" , "As this is entered" , "Astride" , "stride - athwart" .)