Old boy’s ditty about one side of a coin... (7)
I believe the answer is:
obverse
'one side of a coin' is the definition.
(I've seen this before)
'old boy's ditty' is the wordplay.
'old boy' becomes 'ob' (abbreviation).
'ditty' becomes 'verse' (I can't justify this - if you can you should give a lot more credence to this answer).
'ob'+'verse'='OBVERSE'
'about' acts as a link.
Can you help me to learn more?
(Other definitions for obverse that I've seen before include "Side of a coin with the main design" , "Coin's head side" , "Logical proposition" , "Being opposite in verbose way" , "Side of coin that bears head or principle design" .)