"For oft, when on my couch I lie / in ... or in pensive mood" (Wordsworth) (6)

I believe the answer is:

vacant

(Other definitions for vacant that I've seen before include "Horse in a race" , "... Blank" , "blankly incurious" , "Unoccupied - unthinking" , "Of house, not occupied at present" .)

I've seen this clue in The Irish Times.

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