Unappreciated one separates, tense, in temper (13)
I believe the answer is:
misunderstood
'unappreciated' is the definition.
(thesaurus)
'one separates tense in temper' is the wordplay.
'one' becomes 'i' (Roman numeral).
'separates' becomes 'sunders'.
'tense' becomes 't' (abbreviation used in many dictionaries).
'in' is an insertion indicator.
'temper' becomes 'mood' ('mood' can be a synonym of 'temper'**).
'i'+'sunders'+'t'='isunderst'
'isunderst' put into 'mood' is 'MISUNDERSTOOD'.
(Other definitions for misunderstood that I've seen before include "Failed to comprehend" , "Read wrong" , "Failed to construe meaning correctly" , "Failed to see the right meaning" , "Not correctly comprehended" .)