Deep — essential (5)
I believe the answer is:
inner
I believe this is a double definition.
'deep' is the first definition.
(I've seen this before)
'essential' is the second definition.
(thesaurus)
(Other definitions for inner that I've seen before include "Opposite of 13" , "Private, not expressed" , "Internal - mental" , "This sort of man will be satisfied!" , "Part of an archery target" .)