Fitting a support, very large (7)
I believe the answer is:
apropos
'fitting' is the definition.
(thesaurus)
'a support very large' is the wordplay.
'support' becomes 'prop' (prop is a kind of support).
'very large' becomes 'os' (abbreviation for oversize).
'a'+'prop'+'os'='APROPOS'
(Other definitions for apropos that I've seen before include "suitably" , "On the subject of" , "With regard to" , "fitting" , "Poor sap, being to the point" .)