It's not our time, child (5)
I believe the answer is:
their
'it's not our' is the definition.
(I've seen this in another clue)
'time child' is the wordplay.
'time' becomes 't' (abbreviation).
'child' becomes 'heir' (I am not sure about this - if you are sure you should believe this answer much more).
't'+'heir'='THEIR'
Can you help me to learn more?
(Other definitions for their that I've seen before include "Possessive pronoun" , "Some would say his (or her)" , "Those people's" , "Belonging to those people" , "Relating to them" .)