Seed from poppy-head, say (3)
I believe the answer is:
pea
'seed' is the definition.
(I've seen this before)
'poppy-head say' is the wordplay.
'poppy-head' becomes 'p' (head letter of 'poppy-').
'say' shows a homophone (sound like).
'p' is a homophone of 'PEA'.
'from' is the link.
(Other definitions for pea that I've seen before include "Spherical vegetable" , "12 in a pod" , "garden plant" , "It grows in 23 Down" , "Little green vegetable" .)