Dim old boy certain to catch cold (7)
I believe the answer is:
obscure
'dim' is the definition.
(I've seen this before)
'old boy certain to catch cold' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'certain' becomes 'sure' (synonyms).
'to catch' indicates putting letters inside.
'cold' becomes 'c' (eg on taps).
'sure' going around 'c' is 'scure'.
'ob'+'scure'='OBSCURE'
(Other definitions for obscure that I've seen before include "Dull or impenetrable" , "Recondite, abstruse" , "Little known - dark" , "Enigmatic (like Jude?)" , "Dark - secret - inconspicuous" .)