In favour of accepting pound sterling in old money (6)
I believe the answer is:
florin
'old money' is the definition.
(I've seen this before)
'in favour of accepting pound sterling in' is the wordplay.
'in favour of' becomes 'for' (being for a particular cause).
'accepting' is an insertion indicator (inserted letters are accepted).
'pound sterling' becomes 'l' ( is an elaborate letter L).
'for' placed around 'l' is 'flor'.
'flor'+'in'='FLORIN'
(Other definitions for florin that I've seen before include "Old two shilling coin" , "Former English coin" , "Old 2-shilling piece" , "Old two-shilling-piece" , "An old two-shilling coin" .)