National Society for Vegetables (6)
I believe the answer is:
swedes
'vegetables' is the definition.
(swede is a kind of vegetable)
'national society' is the wordplay.
'national' becomes 'swede' (I've seen this before).
'society' becomes 's' (common abbreviation in names of organisations).
'swede'+'s'='SWEDES'
'for' is the link.
(Other definitions for swedes that I've seen before include "Turnips from Scandinavia?" , "Europeans and turnips" , "Root vegetables" , "Turnips for Scandinavians" .)