Too stupid at first to enter even parts of Balliol (4)
I believe the answer is:
also
'too' is the definition.
(synonyms)
'stupid at first to enter even parts of balliol' is the wordplay.
'at first' indicates taking the first letters.
'to enter' means one lot of letters goes inside another.
'even parts of' means one should take alternating letters (the letters in even-numbered positions).
The first letter of 'stupid' is 's'.
The alternate letters of 'balliol' are 'alo'.
's' inserted inside 'alo' is 'ALSO'.
(Other definitions for also that I've seen before include "To boot" , "Additionally" , "on top" , "item's rarely used now for this" , "Plus" .)