Ultimately the morning after, a group of tennis players must go back on the run (2,5)
I believe the answer is:
at large
'on the run' is the definition.
(I've seen this before)
'ultimately the morning after a group of tennis players must go back' is the wordplay.
I cannot really see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
'after' could be 'ala' ('a la' - 'in the style of ' or 'after' in French) and 'ala' is present in the leftover letters.
'group of' could be 'g' (eg the G7 nations, Group of Seven) and 'g' is found within the answer.
The remaining letters 're' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for at large that I've seen before include "Escaped; not yet caught" , "Free, maybe escaped" , "Roaming freely" , "a wild animal may roam so" , "generally" .)