Steep difference, when following old money (7)
I believe the answer is:
pesetas
'old money' is the definition.
(former currency of Spain)
'steep difference when following' is the wordplay.
'difference' indicates an anagram.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'following' says to put letters next to each other.
'steep' with letters rearranged gives 'peset'.
'peset'+'as'='PESETAS'
(Other definitions for pesetas that I've seen before include "Spanish bread no longer used" , "Former currency of Spain" , "cash, once" , "Units of Spanish money" , "Old Spanish money" .)