Way back when? In the end, I couldn't care less (4)
I believe the answer is:
yawn
'i couldn't care less' is the definition.
(a yawn can be an expression of indifference)
'way back when? in the end' is the wordplay.
'back' is a reversal indicator.
'in the end' suggests the final letters.
The final letter of 'when' is 'n'.
'way' back-to-front is 'yaw'.
'yaw'+'n'='YAWN'
(Other definitions for yawn that I've seen before include "Show tiredness/boredom" , "Show one is tired or bored" , "Involuntary sign of tiredness or boredom" , "Boring event" , "Oscitate" .)