Inactive, impassive dog lacking exercise, originally (4)

I believe the answer is:

idle

'inactive' is the definition.
(similar in meaning)

'impassive dog lacking exercise originally' is the wordplay.
'originally' says to take the initial letters.
The initial letters of 'impassive dog lacking exercise' is 'IDLE'.

(Other definitions for idle that I've seen before include "Still" , "Lazy, doing nothing" , "At leisure" , "Dormant" , "Not busy" .)

I've seen this clue in The Times.

Want a hint initially instead of a full solution? Install my app