School nerd retaking sports (12)
I believe the answer is:
kindergarten
'school' is the definition.
(I've seen this before)
'nerd retaking sports' is the wordplay.
'sports' indicates anagramming the letters (I've seen 'sport' mean this).
'nerd'+'retaking'='nerdretaking'
'nerdretaking' with letters rearranged gives 'KINDERGARTEN'.
(Other definitions for kindergarten that I've seen before include "Children's school" , "Pre-school institution for little children" , "Infant school" , "Pre-primary school class" , "Nursery school" .)