Is left to regret being excluded from the side, perhaps (7)
I believe the answer is:
residue
'is left to' is the definition.
I can't judge whether this defines the answer.
'regret being excluded from the side perhaps' is the wordplay.
I cannot quite see how this works, but
an anagram of 'side' is 'esid' which is located in the answer.
'regret' could be 'rue' (to rue something is to regret it) and 'rue' is present in the remaining letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for residue that I've seen before include "Dire use of what is left over" , "Something left over" , "Whatever is left over" , "Material left" , "What's left" .)