Issue of tennis umpire's call that's overturned (4)
I believe the answer is:
emit
'issue of tennis' is the definition.
'emit' can be an answer for 'issue' (both can mean to give out or radiate). I'm not sure about the 'of tennis' bit.
'umpire's call that's overturned' is the wordplay.
'umpire's call' becomes 'time' (I've seen this in another clue).
'that's overturned' says the letters should be written backwards.
'time' reversed gives 'EMIT'.
Can you help me to learn more?
(Other definitions for emit that I've seen before include "Come out!" , "Let out" , "Send out in time" , "Release" , "Make audible sounds" .)