Dim old boy certain to tour college (7)
I believe the answer is:
obscure
'dim' is the definition.
(thesaurus)
'old boy certain to tour college' is the wordplay.
'old boy' becomes 'ob'.
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'to tour' indicates putting letters inside (to tour can mean to go around).
'college' becomes 'c'.
'sure' enclosing 'c' is 'scure'.
'ob'+'scure'='OBSCURE'
(Other definitions for obscure that I've seen before include "Not clear, hidden" , "Gloomy" , "Dull, opaque" , "So Bruce may be dim" , "Overshadow or eclipse" .)