Talks of special vegetables containing potassium (6)
I believe the answer is:
speaks
'talks' is the definition.
('speak' can be a synonym of 'talk')
'special vegetables containing potassium' is the wordplay.
'special' becomes 's' (common abbreviation as in Special Air Service).
'vegetables' becomes 'peas' (I've seen this before).
'containing' indicates putting letters inside.
'potassium' becomes 'k' (K is the chemical symbol for potassium).
'peas' going around 'k' is 'peaks'.
's'+'peaks'='SPEAKS'
'of' acts as a link.
(Another definition for speaks that I've seen is " Utters".)