A steep conversion into old money (6)
I believe the answer is:
peseta
'old money' is the definition.
(I've seen this before)
'a steep conversion' is the wordplay.
'conversion' indicates an anagram.
'a'+'steep'='asteep'
'asteep' is an anagram of 'PESETA'.
'into' is the link.
(Other definitions for peseta that I've seen before include "Old Spanish unit of money" , "One hundred centimos" , "Previous Spanish money unit" , "Get ready once for Spain" , "Old Spanish coin" .)