Out when bound to be heard (6)
I believe the answer is:
asleep
'out' is the definition.
(out can mean unconscious or asleep)
'when bound to be heard' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'bound' suggests removing the centre (I've seen 'bounds of' mean this (the letters which form the boundaries of the word)).
'to be' becomes 'live' (to be is to live or exist).
'heard' becomes 'ep' (I can't justify this - if you can you should give a lot more credence to this answer).
'live' with its centre taken out is 'le'.
'as'+'le'+'ep'='ASLEEP'
Can you help me to learn more?
(Other definitions for asleep that I've seen before include "out like a light?" , "Slumbering" , "Conked out" , "Snoozing" , "Dormant; inattentive" .)