They deliver former US tennis player's son (8)
I believe the answer is:
couriers
'they deliver' is the definition.
'former us tennis player's son' is the wordplay.
'former us tennis player' becomes 'courier' (Jim Courier).
'son' becomes 's'.
'courier'+'s'='COURIERS'
(Other definitions for couriers that I've seen before include "Messengers, now often on motor-bikes" , "They carry messages" , "motorcycle messengers" , "Message carriers" , "Guides" .)