Joint account's now key — loan's expensive, first of all (5)
I believe the answer is:
ankle
'joint' is the definition.
(joint of the body)
'account's now key loan's expensive first of all' is the wordplay.
'first of all' indicates taking the first letters.
The first letters of 'accounts now key loans expensive' is 'ANKLE'.
(Other definitions for ankle that I've seen before include "Major joint in lower limb" , "Part of the leg between the foot and the shin" , "Joint between foot and leg" , "Location of the talus" , "Lower leg joint" .)