Ordered rural men across country C, for one (5,7)
I believe the answer is:
roman numeral
'c for one' is the definition.
(C is the Roman numeral for 100)
'ordered rural men across country' is the wordplay.
'ordered' indicates anagramming the letters.
'across' means one lot of letters goes inside another.
'country' becomes 'oman' (Oman is an example).
'rural'+'men'='ruralmen'
'ruralmen' with letters rearranged gives 'rnumeral'.
'rnumeral' going around 'oman' is 'ROMAN NUMERAL'.
(Other definitions for roman numeral that I've seen before include "Such as V, X or L" , "I say" , "Eg, X or V or M" , "I for one!" , "I, perhaps" .)