Embellish book's jacket, receiving retrospective grant (6)
I believe the answer is:
'embellish' is the definition.
(I know that bedeck is a more specific form of the action embellish)
'book's jacket receiving retrospective grant' is the wordplay.
'jacket' suggests removing the centre (the letters worn by the word as a 'jacket' are use).
'receiving' is an insertion indicator.
'retrospective' says the letters should be written backwards.
'grant' becomes 'cede' (synonyms).
'book' with its centre removed is 'bk'.
'cede' in reverse letter order is 'edec'.
'bk' going around 'edec' is 'BEDECK'.
(Other definitions for bedeck that I've seen before include "dress" , "Festoon, adorn" , "Trim" , "Embellish with adornments" , "Adorn, embellish" .)