Thoroughly apathetic - straight up! (8)
I believe the answer is:
listless
'thoroughly apathetic' is the definition.
'listless' can be an answer for 'apathetic' (I've seen this in another clue). I'm not certain of the 'thoroughly' bit.
'straight up' is the wordplay.
I cannot quite understand how this works, but
'up' could be 'is' (I've seen this in other clues) and 'is' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for listless that I've seen before include "disorganised on a shopping trip?" , "Showing no enthusiasm, torpid" , "Devoid of energy or concern" , "Devoid of energy or interest" , "Lacking zest or vivacity" .)