Line up, set to be cut down? That's less good (5)
I believe the answer is:
worse
'less good' is the definition.
(I've seen this before)
'line up set to be cut down?' is the wordplay.
'line' becomes 'row' (row is a kind of line**).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'set to' becomes 'set' ('set' can be a synonym of 'set to').
'be cut down?' means to remove the last letter (I've seen 'cut down' mean this).
'row' in reverse letter order is 'wor'.
'set' with its final letter removed is 'se'.
'wor'+'se'='WORSE'
'that's' is the link.
(Other definitions for worse that I've seen before include "Of poorer quality" , "Inferior to another" , "More awful or unwell" , "Less favourable" , "More severe; less well" .)