Old people who developed a certain amount of proficiency (5)
I believe the answer is:
iceni
'old people' is the definition.
(ancient tribe of Britain)
'developed a certain amount of proficiency' is the wordplay.
I cannot really see how this works, but
'amount' could be 'en' (en is a kind of amount) and 'en' is present in the answer.
The remaining letters 'ici' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'who' is the link.
Can you help me to learn more?
(Other definitions for iceni that I've seen before include "Boudicca's tribe" , "East Anglian tribe of Britons" , "Ancient British tribe" , "Queen Boudicca's people" , "Ancient tribesmen" .)