Wasted time during last two-thirds of second half? It makes you sick! (6)
I believe the answer is:
emetic
'it makes you sick' is the definition.
(an emetic medicine makes you sick)
'wasted time during last two-thirds of second half?' is the wordplay.
'wasted' indicates anagramming the letters.
'during' means one lot of letters goes inside another.
'last' becomes 'e' (I've seen this in other clues).
'two-thirds of second half?' becomes 'c' (I am not sure about this - if you are sure you should believe this answer much more).
'time' with letters rearranged gives 'meti'.
'e'+'c'='ec'
'meti' going within 'ec' is 'EMETIC'.
(Other definitions for emetic that I've seen before include "medicament" , "It makes one sick" , "Ipecac is one" , "Class including frogs, toads, newts, etc" , "inducing sickness" .)