Once more, at some stage, there's a certain amount of pain (5)
I believe the answer is:
again
'once more' is the definition.
(synonyms)
'some stage there's a certain amount of pain' is the wordplay.
I cannot really see how this works, but
'some' could be 'a' and 'a' is found within the answer.
'there's' could be 'n' (I've seen this in other clues) and 'n' is found within the answer.
'a' is within the answer.
'amount' could be 'gi' (Gi is an example) and 'gi' is found within the remaining letters.
This accounts for all the letters.
This explanation may well be incorrect...
'at' acts as a link.
Can you help me to learn more?
(Other definitions for again that I've seen before include "Repeatedly" , "Moreover - a second time" , "Anew" , "Here we go ___" , "10 10 [AGAIN" .)