Legend or icon, apt to stray (7)
I believe the answer is:
caption
'legend' is the definition.
(synonyms)
'icon apt to stray' is the wordplay.
'to stray' is an anagram indicator.
'icon'+'apt'='iconapt'
'iconapt' is an anagram of 'CAPTION'.
'or' acts as a link.
(Other definitions for caption that I've seen before include "Brief description of an illustration" , "Text under photo" , "Provide (a picture) with a title" , "Legend with a photograph" , "Explanation of an illustration" .)