Educational establishment ruins it every now and then (3)
I believe the answer is:
uni
'educational establishment' is the definition.
(I've seen this before)
'ruins it every now and then' is the wordplay.
'every now and then' indicates alternate letters (letters selected at regular intervals).
'ruins'+'it'='ruinsit'
The alternate letters of 'ruinsit' are 'UNI'.
(Other definitions for uni that I've seen before include "Number of colleges" , "Place of higher education (abbr)" , "College, colloquially" , "Where you can study" , "Reading is one" .)