The stuff of champions in centuries past (5)
I believe the answer is:
since
'past' is the definition.
(I've seen this before)
'the stuff of champions in centuries' is the wordplay.
'the stuff of' indicates a hidden word (I've seen 'stuffing' mean this).
'SINCE' is hidden within 'champions in centuries'.
(Other definitions for since that I've seen before include "From that time one" , "in the past" , "Because; ago" , "Seeing that" .)