I say! You must include old pop star (3,6)
I believe the answer is:
boy george
'pop star' is the definition.
I know nothing about this answer so I can't judge whether this works.
'i say you must include old' is the wordplay.
'i say you' becomes 'bygeorge' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'must include' is an insertion indicator.
'old' becomes 'o' (abbreviation).
'bygeorge' placed around 'o' is 'BOY GEORGE'.
Can you help me to learn more?
(Another definition for boy george that I've seen is " Culture Club singer".)