Let song performed be the most extensive (7)
I believe the answer is:
longest
'the most extensive' is the definition.
(I've seen this in another clue)
'let song performed' is the wordplay.
'performed' indicates an anagram.
'let'+'song'='letsong'
'letsong' is an anagram of 'LONGEST'.
'be' acts as a link.
(Other definitions for longest that I've seen before include "Extending farthest" , "Of greatest duration" , "For the most time" , "Least short" , "Having the most length" .)