It allows a member a certain amount of flexibility (4)
I believe the answer is:
knee
'it allows a member a certain' is the definition.
The definition suggests a singular noun which matches the answer.
'amount of flexibility' is the wordplay.
'amount' becomes 'k' (k is a kind of amount).
'of flexibility' becomes 'nee' (I can't explain this - if you can you should give a lot more credence to this answer).
'k'+'nee'='KNEE'
Can you help me to learn more?
(Other definitions for knee that I've seen before include "Wounded in Indian Wars action" , "One has a cap" , "Leg?joint" , "for going on begging" , "Genuflector?" .)