Birds left tennis riot (7)
I believe the answer is:
linnets
'birds' is the definition.
(linnet is a kind of bird)
'left tennis riot' is the wordplay.
'left' becomes 'l' (common abbreviation).
'riot' indicates anagramming the letters.
'tennis' anagrammed gives 'innets'.
'l'+'innets'='LINNETS'
(Other definitions for linnets that I've seen before include "wingers" , "birds" , "Common finches" , "Would they tweet" .)