Invitation from whistleblower rejected (5)
I believe the answer is:
offer
'invitation' is the definition.
(thesaurus)
'from whistleblower rejected' is the wordplay.
'from' becomes 'of' (of can mean from or belonging to).
'whistleblower' becomes 'ref' (a referee blows a whistle).
'rejected' says the letters should be written in reverse.
'ref' in reverse letter order is 'fer'.
'of'+'fer'='OFFER'
(Other definitions for offer that I've seen before include "show" , "Proposal; reduced price" , "Put forward" , "hand" , "Tender, make bid for" .)