Baby in cooler temperature (6)
I believe the answer is:
infant
'baby' is the definition.
(I know that baby can be written as infant)
'in cooler temperature' is the wordplay.
'cooler' becomes 'fan' (something which cools).
'temperature' becomes 't' (abbreviation).
'in'+'fan'+'t'='INFANT'
(Other definitions for infant that I've seen before include "Wee one" , "Youngster" , "Toddler" , "Young kid" , "Child at an early stage of life" .)