It's clear the old boy has five debts (7)
I believe the answer is:
obvious
'it's clear' is the definition.
(similar in meaning)
'old boy has five debts' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'has' says to put letters next to each other.
'five' becomes 'V' (Roman numeral).
'debts' becomes 'IOUs' (promises to pay debts).
'ob'+'v'+'ious'='OBVIOUS'
'the' acts as a link.
(Other definitions for obvious that I've seen before include "Apparent" , "Conspicuous" , "Glaring" , "Perfectly plain and clear" , "Unmistakable" .)