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11 ac Cut-down strongbox with notes about church bishops? (11)
H?E?S???C?S.
3 d. Sponsored individual one following surgeon into leading position
?O?C???D (8)
24 ac Student song; a mug and I used guitar in arrangement. (9,6)
G?U?E?M?S ?G?T?R
I think this is an anagram of 'a mug' 'I used guitar', perhaps a literary reference which I don't know.

24a: gaudeamus igitur
anagram of "a mug I used guitar"

11a chess pieces (bishops)

ches(t) spie ce s

Godchild (sponsored individual)
one - i
following Dch (surgeon) into
leading position - Gold ?

3, Godchild?

Thank you all for the explanation although I think showing chess pieces as one word a bit naughty, though not uncommon.
The students song caught me out as I don't sing and gave up Latin early