Bacon slice monarch spots first (6)
I believe the answer is:
rasher
'bacon slice' is the definition.
(I've seen this before)
'monarch spots first' is the wordplay.
I cannot really understand how this works, but
'monarch' could be 'er' (abbreviation for Elizabeth Regina, British Queen) and 'er' is present in the answer.
'spots' could be 'rash' (I've seen this before) and 'rash' is found in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for rasher that I've seen before include "pig, to an extent" , "Sliver of bacon" , "Less considered" , "breakfast selection?" , "Less circumspect" .)