Humble old boy sure to catch cold (7)
I believe the answer is:
obscure
'humble' is the definition.
(thesaurus)
'old boy sure to catch cold' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'to catch' indicates putting letters inside.
'cold' becomes 'c' (eg on taps).
'sure' placed around 'c' is 'scure'.
'ob'+'scure'='OBSCURE'
(Other definitions for obscure that I've seen before include "Little-known" , "Conceal; uncertain" , "Dim, indistinct or not well known" , "Dim, vague or not well known" , "Not clear, indistinct" .)