Favour whistleblower according to guards (6)
I believe the answer is:
prefer
'favour' is the definition.
('prefer' can be a synonym of 'favour')
'whistleblower according to guards' is the wordplay.
'whistleblower' becomes 'ref' (referee).
'according to' becomes 'per' (I've seen this before).
'guards' means one lot of letters goes inside another.
'ref' put within 'per' is 'PREFER'.
(Other definitions for prefer that I've seen before include "support" , "Advance" , "Promote; like better" , "Choose as more desirable, like better" , "Choose over others" .)