Principle that is absorbing Democrat completely? Almost! (5)
I believe the answer is:
ideal
'principle' is the definition.
(I've seen this before)
'that is absorbing democrat completely? almost' is the wordplay.
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'absorbing' means one lot of letters goes inside another.
'democrat' becomes 'd'.
'completely?' becomes 'all' (synonyms).
'almost' means to remove the last letter (almost the whole word is kept).
'ie' going around 'd' is 'ide'.
'all' with its last letter removed is 'al'.
'ide'+'al'='IDEAL'
(Other definitions for ideal that I've seen before include "Perfect for the purpose" , "Model of excellence (5)." , "that can't be bettered?" , "A deli (anag.)" , "fine" .)