Child in time keeps cooler (6)
I believe the answer is:
infant
'child' is the definition.
(I know that infant is a type of child)
'in time keeps cooler' is the wordplay.
'time' becomes 't' (abbreviation).
'keeps' means one lot of letters goes inside another (inserted letters are kept inside).
'cooler' becomes 'fan' (something which cools).
'in'+'t'='int'
'int' going around 'fan' is 'INFANT'.
(Other definitions for infant that I've seen before include "Juvenile" , "Child at an early stage of life" , "Youngster" , "Small baby" , "Small boy" .)