Once the cost of giving Mad Rolf short measure! (6)
I believe the answer is:
florin
'once the cost of' is the definition.
I can't tell whether this definition defines the answer.
'mad rolf short measure' is the wordplay.
'mad' indicates an anagram.
'short' means to remove the last letter.
'measure' becomes 'ins'.
'ins' with its final letter removed is 'in'.
'rolf' anagrammed gives 'flor'.
'flor'+'in'='FLORIN'
'giving' is the link.
Can you help me to learn more?
(Other definitions for florin that I've seen before include "Old 2-shilling piece" , "Old silver coin" , "An old two-shilling coin" , "Former British silver coin" , "Old two-shilling-piece" .)