Less widely known rule on poison copper ignored (5)
I believe the answer is:
rarer
'poison copper ignored' is the definition.
I can't judge whether this definition defines the answer.
'less widely known rule on' is the wordplay.
I cannot quite understand how this works, but
'rule' could be 'r' (legal abbreviation) and 'r' is located in the answer.
'on' could be 're' ('on' can be mean 'with reference to') and 're' is found in the answer.
The remaining letters 'ar' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for rarer that I've seen before include "Thinner" , "Less frequent" , "(Of meat) less cooked" , "Stranger" , "More scarce or uncommon" .)