An age in school? Time to escape! (3)
I believe the answer is:
eon
'an age' is the definition.
(I know that eon is a type of age)
'school? time to escape' is the wordplay.
'school?' becomes 'eton' (famous UK public school).
'time' becomes 't' (abbreviation).
'to escape' indicates named letters should be taken away.
'eton' with 't' taken away is 'EON'.
'in' is the link.
(Other definitions for eon that I've seen before include "In time, long division" , "Major division of geological time" , "Long period of time" , "Vast age (alt. sp.)" , "Vast period" .)