Celt Dee unexpectedly chose (7)
I believe the answer is:
elected
'chose' is the definition.
(electing is a kind of choosing)
'celt dee unexpectedly' is the wordplay.
'unexpectedly' is an anagram indicator.
'celt'+'dee'='celtdee'
'celtdee' with letters rearranged gives 'ELECTED'.
(Other definitions for elected that I've seen before include "Chosen for office" , "Appointed" , "Voted into office" , "Chosen by vote" , "Chosen by ballot" .)