Two-time champion's beginning to warm up (5)
I believe the answer is:
cheat
'two-time' is the definition.
(I've seen this before)
'champion's beginning to warm up' is the wordplay.
'beginning to' indicates taking the first letters.
'warm up' becomes 'heat' (I've seen this before).
The first letter of 'champions' is 'c'.
'c'+'heat'='CHEAT'
(Other definitions for cheat that I've seen before include "Two-time" , "Take unfair advantage of" , "Hoodwink" , "Act dishonestly to gain an advantage" , "Deceive, defraud" .)