Nobody's free to swap spectacles for monocle 'by right' (5)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

“loose”(free/unattached) with “oo”(letters shaped like a pair of glasses/spectacles) replaced by ( to swap … for … ) “o”(letter shaped like a monocle) plus ( by ) R(abbrev. for “right”).