Serb sank men's first and second seeds (6)
I believe the answer is:
sesame
'serb sank' is the definition.
I can't tell whether this defines the answer.
'men's first and second seeds' is the wordplay.
'men' becomes 'se' (I have seen 'Spanish men ' mean 'se' so perhaps 'men' could also mean 'se').
'first and' becomes 'sam' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'second seeds' becomes 'e' (and letter of 'seeds').
'se'+'sam'+'e'='SESAME'
(Other definitions for sesame that I've seen before include "Plant cultivated for seeds providing oil" , "Source of seed and oil" , "Open .... is a pantomime cry" , "A seed; a TV Street" , "Ali Baba's opening command" .)