Suggestion of upsetting the referee (5)
I believe the answer is:
offer
'suggestion' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'of upsetting the referee' is the wordplay.
'upsetting' says the letters should be written backwards.
'the referee' becomes 'ref' (synonyms).
'ref' in reverse letter order is '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" .)