Old boy sure to round cape in dark (7)
I believe the answer is:
obscure
'dark' is the definition.
(I know that dark can be written as obscure)
'old boy sure to round cape' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'to round' indicates putting letters inside.
'cape' becomes 'c' (abbreviation used on maps).
'sure' going around 'c' is 'scure'.
'ob'+'scure'='OBSCURE'
'in' is the link.
(Other definitions for obscure that I've seen before include "Indistinct or not well-known" , "not easily understood" , "Conceal; uncertain" , "incomprehensible" , "Jude may be" .)