Hard growth is essentially difficult (4)
I believe the answer is:
horn
'hard growth' is the definition.
(hard growth from an animal's head)
'essentially difficult' is the wordplay.
'essentially' says to take the centre.
'difficult' becomes 'thorny' ('thorny' can be similar in meaning to 'difficult').
The middle of 'thorny' is 'HORN'.
'is' acts as a link.
(Other definitions for horn that I've seen before include "Hollow outgrowth and musical instrument" , "Hooter" , "being blown in the wind?" , "Warning device on a vehicle" , "Possession of huntsman or deer" .)