Dull with sleep, past taking in half a play (6)
I believe the answer is:
bleary
'dull with sleep' is the definition.
(I've seen this before)
'past taking in half a play' is the wordplay.
'past' becomes 'by' (as in years gone by).
'taking in' indicates putting letters inside.
'half a' suggests halving the letters.
'play' becomes 'King Lear' (Shakespeare play).
'kinglear' halved is 'lear'.
'by' placed around 'lear' is 'BLEARY'.
(Other definitions for bleary that I've seen before include "not clear" , "(Of eyes) dull and un-focussed from sleep" , "Smudged, indistinct" , "Unable to see properly" , "Eyes like this are dull from exhaustion" .)