Support intention to retain singular old expression (8)
I believe the answer is:
archaism
'old expression' is the definition.
'support intention to retain singular' is the wordplay.
'support' becomes 'arch'.
'intention' becomes 'aim' (synonyms).
'to retain' is an insertion indicator.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'aim' enclosing 's' is 'aism'.
'arch'+'aism'='ARCHAISM'
(Other definitions for archaism that I've seen before include "Outdated word" , "Old-fashioned expression" , "Something outdated" , "Something in an out of date style" , "Ancient word or expression for charisma" .)