Allow into commercial college (5)
I believe the answer is:
admit
'allow' is the definition.
(admit can mean to allow or accept)
'commercial college' is the wordplay.
'commercial' becomes 'ad' (abbreviation for advertisement).
'college' becomes 'MIT' (Massachusetts Institute of Technology).
'ad'+'mit'='ADMIT'
'into' acts as a link.
(Other definitions for admit that I've seen before include "Allow entry" , "Confess (to something)" , "Grant entry to" , "Allow access" , "Recognise" .)