Waste time on skin condition (5)
I believe the answer is:
trash
'waste' is the definition.
(I know that waste can be written as trash)
'time on skin condition' is the wordplay.
'time' becomes 't' (abbreviation).
'on' says to put letters next to each other.
'skin condition' becomes 'rash' (I've seen this in another clue).
't'+'rash'='TRASH'
(Other definitions for trash that I've seen before include "Refuse, waste" , "Rubbish, worthless stuff" , "bunk" , "Waste material (US)" , "Wreck; rubbish" .)