If idle men stray here it could be fatal
I believe the answer is:
minefield
'here it could be fatal' is the definition.
The answer is a location as well as being a singular noun. This is suggested by the definition.
'if idle men stray' is the wordplay.
'stray' is an anagram indicator.
'if'+'idle'+'men'='ifidlemen'
'ifidlemen' is an anagram of 'MINEFIELD'.
Can you help me to learn more?
(Other definitions for minefield that I've seen before include "Sensitive area" , "Highly delicate situation (where one must watch one's step!)" , "Danger zone" , "Dangerous area" .)