Joint of deer not available, needing replacing (5)
I believe the answer is:
ankle
'joint' is the definition.
(ankle is a kind of joint)
'deer not available needing replacing' is the wordplay.
'deer' becomes 'elk' (elk is a kind of deer).
'not available' becomes 'na' (abbreviation for not available).
'needing replacing' says the letters should be written in reverse (letters placed in the opposite direction).
'elk'+'na'='elkna'
'elkna' reversed gives 'ANKLE'.
'of' acts as a link.
(Other definitions for ankle that I've seen before include "Joint between foot and leg" , "Major foot joint" , "Part of the leg between the foot and the shin" , "Location of the talus" , "Low joint" .)