Extremely powerful tradition at heart (5)
I believe the answer is:
ultra
'extremely' is the definition.
(prefix meaning extremely)
'powerful tradition at heart' is the wordplay.
'at heart' says the answer is hidden in the clue (letters hidden in the heart of the clue).
'ULTRA' is hidden in the letters of 'powerful tradition'.
(Other definitions for ultra that I've seen before include "Saharan dweller" , "Prefix meaning excessively or extremely" , "Prefix meaning beyond or extremely" , "Violet's predecessor" , "extreme measures" .)