15 down
Endorsement
after
president
would
indicate
top
choice
(9)
Endorsement after president would indicate top choice (9)
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The definition is at the beginning of the clue.
How the clue works
Endorsement defines REFERENCE, the kind you give someone. The wordplay notes that adding P (president) in front of the answer would give PREFERENCE, meaning "top choice" — so REFERENCE is PREFERENCE with its leading P removed.