Spanish monetary unit (6)
I believe the answer is:
peseta
'spanish monetary unit' is the definition.
(peseta is a kind of spanish monetary unit)
This is all the clue.
(Other definitions for peseta that I've seen before include "Spain's former currency unit" , "Pre-Euro Spanish currency" , "currency being superseded" , "Previous Spanish money unit" , "Formerly the basic unit of money in Spain" .)