Infant born during recess (4)
I believe the answer is:
baby
'infant' is the definition.
(synonyms)
'born during recess' is the wordplay.
'born' becomes 'b'.
'during' is an insertion indicator.
'recess' becomes 'bay' (bay is a kind of recess).
'b' placed into 'bay' is 'BABY'.
(Other definitions for baby that I've seen before include "One born" , "Mollycoddle" , "Person born just after WWII" , "needing dummy?" , "Newly-born child" .)