Deck in mayhem? Bell is hammered (9)
I believe the answer is:
embellish
'deck' is the definition.
(thesaurus)
'in mayhem? bell is hammered' is the wordplay.
'in' indicates a hidden word.
'EMBELLISH' is hidden in the letters of 'mayhem bell is hammered'.
(Other definitions for embellish that I've seen before include "Add ornamental details, make more colourful" , "Make more interesting by adding details that may be untrue" , "He bills me so as to improve or decorate" , "Improve story by adding invented touches" , "Make more attractive by adding details" .)