Oscar felt terrible supporting very professional U-turn (5-4)
I believe the answer is:
volte-face
'u-turn' is the definition.
(I've seen this before)
'oscar felt terrible supporting very professional' is the wordplay.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'terrible' indicates an anagram.
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'very' becomes 'v' (common abbreviation).
'professional' becomes 'ace'.
'felt' anagrammed gives 'ltef'.
'o'+'ltef'='oltef'
'oltef' after 'v' is 'voltef'.
'voltef'+'ace'='VOLTE-FACE'
(Other definitions for volte-face that I've seen before include "A complete chnage of mind or attitude" , "A complete change of mind or attitude" , "Complete or sudden change of attitude" , "Complete change of attitude from French" , "About-face, major change of attitude" .)