Demand little sibling enters in time (6)
I believe the answer is:
insist
'demand' is the definition.
(to insist on something is to demand it)
'little sibling enters in time' is the wordplay.
'little sibling' becomes 'sis' (short for sister).
'enters' is an insertion indicator.
'time' becomes 't' (abbreviation).
'in'+'t'='int'
'sis' placed within 'int' is 'INSIST'.
(Other definitions for insist that I've seen before include "Declare or demand emphatically" , "Demand forcefully" , "Maintain persistently" , "Be firmly demanding" , "Put one's foot down" .)