Mustard? You might pick up the seed! (7)
I believe the answer is:
colonel
'mustard?' is the definition.
(Cluedo character Colonel Mustard)
'you might pick up the seed' is the wordplay.
'you might pick up' shows a homophone (sound like) (pick up can mean to hear).
'the seed' becomes 'kernel' (kernel is a kind of seed).
'kernel' sounds like 'COLONEL'.
(Other definitions for colonel that I've seen before include "Army officer ranking below a brigadier" , "'No cello for this officer, strangely (7)'" , "Army rank" , "Comissioned military officer" , "Commissioned officer" .)