A number welcoming opening of tennis courts (5)
I believe the answer is:
atria
'courts' is the definition.
(atrium is a kind of court)
'a number welcoming opening of tennis' is the wordplay.
'a number' becomes 'aria' (I have seen 'Operatic number ' mean 'aria' so perhaps 'number' could also mean 'aria').
'welcoming' is an insertion indicator (in sense of 'inviting in').
'opening of' indicates taking the first letters.
The initial letter of 'tennis' is 't'.
'aria' placed around 't' is 'ATRIA'.
(Other definitions for atria that I've seen before include "Covered courts" , "Open courts of Roman houses" , "Open-roofed courts" , "Central areas in buildings; open to the sky" , "entrances" .)