Old coin placed inside seed (6)
I believe the answer is:
peseta
'old coin' is the definition.
(I've seen this before)
'placed inside seed' is the wordplay.
'placed' becomes 'set' (synonyms).
'inside' is an insertion indicator.
'seed' becomes 'pea' (I've seen this before).
'set' placed within 'pea' is 'PESETA'.
(Other definitions for peseta that I've seen before include "Formerly, unit of Spanish money" , "Formerly the basic unit of money in Spain" , "Domestic social" , "Spanish bread once" , "Former monetary unit of Spain" .)