One in Rolls makes merry (8)
I believe the answer is:
roisters
'makes merry' is the definition.
(roistering is a kind of making merry)
'one in rolls' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
'rolls' could be 'rosters' ('roster' can be a synonym of 'roll') and 'rosters' is present in the remaining letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for roisters that I've seen before include "[WASSAILS]" , "Parties" , "Makes merry in sorriest way" , "Noisily celebrates" , "Carouses" .)