Old people’s reserve, popular when brought up (5)
I believe the answer is:
iceni
'old people's' is the definition.
(I've seen this before)
'reserve popular when brought up' is the wordplay.
'reserve' becomes 'ice' (I've seen this before).
'popular' becomes 'in'.
'when brought up' is a reversal indicator.
'in' back-to-front is 'ni'.
'ice'+'ni'='ICENI'
(Other definitions for iceni that I've seen before include "Ancient tribesmen" , "Boudica's tribe" , "East Anglian tribe of Britons" , "Rebellious tribe" , "Boadicea's tribe" .)