A long time in Berkshire college — wasting time! (3)
I believe the answer is:
eon
'a long time' is the definition.
(eon is a kind of long time)
'berkshire college wasting time' is the wordplay.
'berkshire college' becomes 'eton' (I've seen this before).
'wasting' is a deletion indicator.
'time' becomes 't' (abbreviation).
'eton' with 't' taken out is 'EON'.
'in' is the link.
(Other definitions for eon that I've seen before include "In time, long division" , "Vast age (alt. sp.)" , "Age, epoch" , "Geological period" , "thousand million years" .)