Preschool carer -- really heartless (7)
I believe the answer is:
nursery
'preschool' is the definition.
(thesaurus)
'carer really heartless' is the wordplay.
'carer' becomes 'nurse' (I've seen this before).
'heartless' says to hollow out the word (remove centre letters).
'really' with its middle removed is 'ry'.
'nurse'+'ry'='NURSERY'
(Other definitions for nursery that I've seen before include "Room for baby or baby plant" , "School for young children" , "An end of some tests" , "Race or place for two-year-olds" , "where small fry are found" .)