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