... again, but for the last time? (4,4)

'again' is the definition.
('once more' can be a synonym of 'again')

'for the last time?' is the wordplay.
I cannot quite understand how this works, but
'time?' could be 'once' (I have seen 'Single time ' mean 'once' so perhaps 'time' could also mean 'once') and 'once' is present in the answer.
The remaining letters 'more' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

'but' is the link.

Can you help me to learn more?