Suggestion of upsetting the referee (5)
I believe the answer is:
offer
'suggestion' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'of upsetting the referee' is the wordplay.
'upsetting' says the letters should be written in reverse.
'the referee' becomes 'ref' ('ref' can be a synonym of 'referee').
'ref' written backwards gives 'fer'.
'of'+'fer'='OFFER'
Can you help me to learn more?
(Other definitions for offer that I've seen before include "(Suggestion of) gift" , "Tender, volunteer" , "Come up with" , "Pay increase" , "Submit" .)