At best, twenty? It may well be more! (3,5)
I believe the answer is:
top score
'it may well be more' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'at best twenty?' is the wordplay.
'at' means one lot of letters go next to another.
'best' becomes 'top' (top can mean the very best).
'twenty?' becomes 'score' (synonyms).
'top'+'score' is 'TOP SCORE'.
Can you help me to learn more?
(Another definition for top score that I've seen is " one hundred and eighty!".)