Secretly bet a rival's return would make them furious (5)
I believe the answer is:
irate
'furious' is the definition.
(I've seen this in another clue)
'secretly bet a rival's return' is the wordplay.
'secretly' says the answer is hidden in the clue.
'return' says the letters should be written backwards.
'etari' is hidden within 'bet a rivals'.
'etari' backwards is 'IRATE'.
'would make them' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for irate that I've seen before include "Fuming" , "Not happy" , "Angry, annoyed" , "Annoyed, nettled" , "Ticked off" .)