The legend of officer no one backed (7)
I believe the answer is:
caption
'the legend' is the definition.
('caption' can be a synonym of 'legend')
'officer no one backed' is the wordplay.
'officer' becomes 'capt' (**).
'one' becomes 'i' (Roman numeral).
'backed' says the letters should be written in reverse.
'no'+'i'='noi'
'noi' reversed gives 'ion'.
'capt'+'ion'='CAPTION'
'of' is the link.
(Other definitions for caption that I've seen before include "Explanation of an illustration" , "Explanatory words or heading" , "Brief description of an illustration" , "Provide (a picture) with a title" , "Text under photo" .)