Unit of distance? More like time, stupid! (9)
I believe the answer is:
kilometre
'unit of distance?' is the definition.
(I've seen this before)
'more like time stupid' is the wordplay.
'time' becomes 't'.
'stupid' indicates anagramming the letters (in a stupid order).
'more'+'like'+'t'='moreliket'
'moreliket' anagrammed gives 'KILOMETRE'.
(Other definitions for kilometre that I've seen before include "Metrical measure of 0.62 miles" , "Unit of distance" , "Measure of distance" , "K stands for this" , "Unit of length" .)