Vegetable bringing in ready money once (6)
I believe the answer is:
peseta
'money once' is the definition.
(former Spanish currency)
'vegetable bringing in ready' is the wordplay.
'vegetable' becomes 'pea' (pea is a kind of vegetable**).
'bringing in' indicates putting letters inside.
'ready' becomes 'set' ('set' can be similar in meaning to 'ready').
'pea' placed around 'set' is 'PESETA'.
(Other definitions for peseta that I've seen before include "Pre-euro money unit in Spain" , "Former Spanish monetary unit" , "little cash is required" , "Formerly, unit of Spanish money" , "Previous Spanish money unit" .)