The warning light was to be a scam (6)
I believe the answer is:
beacon
'the warning light' is the definition.
(I've seen this before)
'to be a scam' is the wordplay.
'scam' becomes 'con' (I've seen this before).
'be'+'a'+'con'='BEACON'
'was' acts as a link.
(Other definitions for beacon that I've seen before include "Signal fire - lighthouse" , "Warning light on hilltop" , "Signal fire - signalling buoy" , "Light on a height for navigation or warning" , "Fire on a hill, lit as a signal" .)