Essential sign in heart oddly ignored (5)
I believe the answer is:
inner
'essential' is the definition.
(thesaurus)
'sign in heart oddly ignored' is the wordplay.
'oddly ignored' means one should take alternating letters (letters in odd-numbered positions are removed).
'sign'+'in'+'heart'='signinheart'
The alternating letters of 'signinheart' are 'INNER'.
(Other definitions for inner that I've seen before include "closer than magpie" , "Esoteric" , "Inside; next to the bull (archery)" , "Very private room" , "Archers know about this" .)