Briefly soothe copper with fifty one gallstones? (7)
I believe the answer is:
calculi
'gallstones?' is the definition.
(gallstone is a kind of calculus)
'briefly soothe copper with fifty one' is the wordplay.
'briefly' means to remove the last letter.
'soothe' becomes 'calm' (I've seen this before).
'copper' becomes 'cu' (chemical symbol).
'with' is a charade indicator (letters next to each other).
'fifty one' becomes 'li' (Roman numeral).
'calm' with its final letter taken away is 'cal'.
'cal'+'cu'+'li'='CALCULI'
(Another definition for calculi that I've seen is " gallstones?".)