Ron and I fret about marches (8)
I believe the answer is:
frontier
'marches' is the definition.
(thesaurus)
'ron and i fret about' is the wordplay.
'and' means one lot of letters go next to another (I've seen this in other clues).
'about' is an anagram indicator.
'i'+'fret'='ifret'
'ron' put after 'ifret' is 'ifretron'.
'ifretron' is an anagram of 'FRONTIER'.
(Other definitions for frontier that I've seen before include "Verge" , "Marches" , "Border (not yet crossed?)" , "Border separating two countries" , "Boundary, border" .)