Bone needing potassium -- nurse on the counter putting measurement up (7)
I believe the answer is:
kneecap
'bone' is the definition.
(I know that kneecap is a type of bone)
'potassium nurse on the counter putting measurement up' is the wordplay.
'potassium' becomes 'K' (K is the chemical symbol for potassium).
'nurse' becomes 'en' (abbreviation for enrolled nurse).
'on the counter' is a reversal indicator (counter can mean in the opposite direction).
'putting measurement up' becomes 'ecap' (I can't explain this - if you can you should give a lot more credence to this answer).
'en' backwards is 'ne'.
'k'+'ne'+'ecap'='KNEECAP'
'needing' is the link.
Can you help me to learn more?
(Other definitions for kneecap that I've seen before include "The informal patella (7)" , "Joint protection" , "one going on bender?" , "Bone" , "Horribly punish" .)