With luck, Ken’s rolling a joint (7)
I believe the answer is:
knuckle
'a joint' is the definition.
(joint in the hand)
'with luck ken's rolling' is the wordplay.
'with' says to put letters next to each other.
'rolling' indicates anagramming the letters.
'luck'+'ken' is 'luckken'.
'luckken' is an anagram of 'KNUCKLE'.
(Other definitions for knuckle that I've seen before include "Hand-joint" , "Bone of the fist" , "Knob at finger joint" , "Hand joint" , "& 22 Down Yield to pressure" .)