Work feverishly at tennis, if Open's beginning (11)
I believe the answer is:
sinfonietta
'work' is the definition.
(I've seen this before)
'feverishly at tennis if open's beginning' is the wordplay.
'feverishly' is an anagram indicator (feverish can mean wild or agitated).
'beginning' suggests taking the first letters.
The first letter of 'opens' is 'o'.
'at'+'tennis'+'if'+'o'='attennisifo'
'attennisifo' with letters rearranged gives 'SINFONIETTA'.
(Other definitions for sinfonietta that I've seen before include "Small orchestral piece" , "group of players" , "Small orchestra" , "work" .)