Heard substantial chunk of Time is owned by US? (4)
I believe the answer is:
ours
'owned by us?' is the definition.
(something which is ours belongs to us)
'heard substantial chunk of time' is the wordplay.
'heard' shows a homophone (sound like).
'substantial chunk of time' becomes 'hours' (several hours).
'hours' sounds like 'OURS'.
'is' acts as a link.
(Other definitions for ours that I've seen before include "Indiscriminate slaughter" , "of the Queen?" , "What we share" , "Relating or belonging to us" , "British" .)