We put in hardened seed to produce plant (5,3)
I believe the answer is:
sweet pea
'plant' is the definition.
(sweet pea is a kind of plant)
'we put in hardened seed' is the wordplay.
'put in' indicates putting letters inside.
'hardened' becomes 'set' ('set' can be a synonym of 'hardened').
'seed' becomes 'pea' (I've seen this before).
'we' inserted inside 'set' is 'sweet'.
'sweet'+'pea'='SWEET PEA'
'to produce' is the link.
(Other definitions for sweet pea that I've seen before include "Climbing pant" , "Perfumed climber" , "Fragrant flower" , "Fragrant climber" , "Peseta we paid for climbing garden flower" .)