Adult support facing very big fitting (7)
I believe the answer is:
apropos
'fitting' is the definition.
(thesaurus)
'adult support facing very big' is the wordplay.
'adult' becomes 'a'.
'support' becomes 'prop' (prop is a kind of support**).
'facing' is a charade indicator (letters next to each other).
'very big' becomes 'os' (abbreviation for outsize).
'a'+'prop'+'os'='APROPOS'
(Other definitions for apropos that I've seen before include "Touching" , "To the point about poor spa" , "Poor sap, being to the point" , "Suitable or relevant" , "As regards" .)