Regional dialect bringing cash to Duck Island (6)
I believe the answer is:
patois
'regional dialect' is the definition.
(I've seen this before)
'cash to duck island' is the wordplay.
'cash' becomes 'Pat' (tennis player Pat Cash).
'to duck' becomes 'o' (resembles zero - 'duck' in cricket).
'island' becomes 'is' (abbreviation).
'pat'+'o'+'is'='PATOIS'
'bringing' acts as a link.
(Other definitions for patois that I've seen before include "'Argot, vernacular (6)'" , "A regional dialect of a language" , "Kind of dialect, from French" , "manner of speaking" , "speech" .)