Principle that is engaging Democrat most of all (5)
I believe the answer is:
ideal
'principle' is the definition.
(thesaurus)
'that is engaging democrat most of all' is the wordplay.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'engaging' means one lot of letters goes inside another.
'democrat' becomes 'd'.
'most of' means to remove the last letter.
'ie' enclosing 'd' is 'ide'.
'all' with its final letter taken off is 'al'.
'ide'+'al'='IDEAL'
(Other definitions for ideal that I've seen before include "Ailed (anag)" , "Model of ethical behaviour" , "existing only in imagination" , "Perfectly suited" , "Standard of perfection or excellence" .)