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 judge whether this defines the answer.
'mad rolf short measure' is the wordplay.
'mad' indicates anagramming the letters.
'short' means to remove the last letter.
'measure' becomes 'ins'.
'ins' with its last letter taken off is 'in'.
'rolf' is an anagram of 'flor'.
'flor'+'in'='FLORIN'
'giving' acts as a 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" .)