Mark lifted M&S confectionery, losing head (6)
I believe the answer is:
smudge
'mark' is the definition.
(I've seen this before)
'lifted ms confectionery losing head' is the wordplay.
'lifted' says the letters should be written in reverse (going up in down clue).
'confectionery' becomes 'fudge' (fudge is a kind of confectionery).
'losing head' means to remove the first letter.
'ms' backwards is 'sm'.
'fudge' with its first letter taken off is 'udge'.
'sm'+'udge'='SMUDGE'
(Other definitions for smudge that I've seen before include "Blur, smear" , "blurred impression" , "Soil - daub" , "A smeared mark or stain" , "A dirty or blurred mark" .)