At college, a small tree (4)
I believe the answer is:
upas
'tree' is the definition.
(upas is a species of tree)
'at college a small' is the wordplay.
'at college' becomes 'up'.
'small' becomes 's' (abbreviation - e.g. clothes size).
'up'+'a'+'s'='UPAS'
(Other definitions for upas that I've seen before include "Tree from Java with poisonous sap" , "Tree yielding poisonous sap" , "Source of poison" , "A poisoner" , "Sap used to poison arrows" .)