A seed that grows into a tree (5)
I believe the answer is:
acorn
'a seed that grows into a tree' is the definition.
'acorn' can be an answer for 'a seed' (I've seen this before). I'm not certain of the rest of the definition.
'a seed' is the wordplay.
'seed' becomes 'corn' (corn is a kind of seed).
'a'+'corn'='ACORN'
Can you help me to learn more?
(Other definitions for acorn that I've seen before include "from which something bigger may develop" , "Nut of oak tree" , "Might source" , "Tree seed" , "Source of a great oak" .)