Formal jacket fixed centrally during day before party (6)
I believe the answer is:
tuxedo
'formal jacket' is the definition.
'fixed centrally during day before party' is the wordplay.
'centrally' says to take the centre.
'during' indicates putting letters inside.
'day' becomes 'tue' (**).
'before' says to put letters next to each other.
'party' becomes 'do' (do is a kind of party).
The centre of 'fixed' is 'x'.
'x' going within 'tue' is 'tuxe'.
'tuxe'+'do'='TUXEDO'
(Other definitions for tuxedo that I've seen before include "American DJ" , "Garment" , "An American monkey-suit" , "American dinner jacket" , "Dinner jacket (US)" .)