Decking gran, I nod wickedly (8)
I believe the answer is:
adorning
'decking' is the definition.
(adorn can mean to deck or decorate)
'gran i nod wickedly' is the wordplay.
'wickedly' is an anagram indicator.
'gran'+'i'+'nod'='graninod'
'graninod' with letters rearranged gives 'ADORNING'.
(Other definitions for adorning that I've seen before include "Decorating with ornaments" , "decking" , "Decorating, embellishing" .)