Rasher guards last to observe warning signal (6)
I believe the answer is:
beacon
'warning signal' is the definition.
'beacon' can be an answer for 'warning' (I've seen this before). I also know that 'signal' relates to this answer.
'rasher guards last to observe' is the wordplay.
'rasher' becomes 'bacon' (I've seen this before).
'guards' indicates putting letters inside.
'last to' suggests the final letters.
The last letter of 'observe' is 'e'.
'bacon' placed around 'e' is 'BEACON'.
(Other definitions for beacon that I've seen before include "Fire on a hill, lit as a signal" , "Warning light on hill or cliff" , "This may serve as a warning" , "guidance" , "Guilding light" .)