A burden? Not this sort of clue! (6)
I believe the answer is:
across
'not this sort of clue' is the definition.
'across' can be an answer for 'sort of clue' (I've seen this before). I'm not certain of the rest of the definition.
'a burden?' is the wordplay.
'burden?' becomes 'cross' (someone's particular burden is their cross).
'a'+'cross'='ACROSS'
Can you help me to learn more?
(Other definitions for across that I've seen before include "Transverse" , "stride - athwart" , "From side to side, like this clue" , "written like this?" , "Clues like this" .)