A hard growth seen on oak (5)
I believe the answer is:
acorn
'on oak' is the definition.
The answer and definition can be both plants as well as being singular nouns.
I also know that 'oak' relates to this answer.
'a hard growth' is the wordplay.
'hard growth' becomes 'corn' (I have seen 'digital growth' mean 'corn' so perhaps 'growth' could also mean 'corn'. I am not sure about the 'hard' bit.).
'a'+'corn'='ACORN'
'seen' is the link.
Can you help me to learn more?
(Other definitions for acorn that I've seen before include "Dirt on a liquid surface" , "From which a tall tree may grow" , "Fruit of the oak tree" , "Potential oak" , "oak's fruit" .)