Old people can be cool . . . flipping cool (5)
I believe the answer is:
iceni
'old people' is the definition.
(ancient tribe of Britain)
'cool flipping cool' is the wordplay.
'cool' becomes 'ice' (icing is a kind of cooling).
'flipping' says the letters should be written in reverse.
'cool' becomes 'in' ('in' can mean fashionable or cool).
'in' in reverse letter order is 'ni'.
'ice'+'ni'='ICENI'
'can be' acts as a link.
(Other definitions for iceni that I've seen before include "Rebellious tribe" , "Old warriors" , "Boadicea's tribe" , "Ancient Brits" , "Boudica's tribe" .)