Substitute what tennis players often do
I believe the answer is:
reserve
'substitute' is the definition.
(I've seen this before)
'often do' is the wordplay.
'often' becomes 're' (I can't explain this - if you can you should believe this answer much more).
'do' becomes 'serve' (serving is a kind of doing).
're'+'serve'='RESERVE'
'what tennis players' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for reserve that I've seen before include "Hold back" , "Keep for someone - spare bod" , "Keep for future use" , "Wine kept" , "Keep back - book" .)