Sing terrible corn about nothing (5)
I believe the answer is:
croon
'sing' is the definition.
(crooning is a kind of singing)
'terrible corn about nothing' is the wordplay.
'terrible' indicates anagramming the letters.
'about' means one lot of letters goes inside another.
'nothing' becomes 'o' (looks like zero - 0).
'corn' with letters rearranged gives 'cron'.
'cron' enclosing 'o' is 'CROON'.
(Other definitions for croon that I've seen before include "Sing like Bing" , "Sing softly" , "Sing in a soft low tone" , "Sing with a soft voice" .)