A support thus erected is suitable (7)
I believe the answer is:
apropos
'suitable' is the definition.
(apropos means appropriate or pertinent)
'a support thus erected' is the wordplay.
'support' becomes 'prop' (prop is a kind of support).
'thus' becomes 'so' (synonyms).
'erected' is a reversal indicator (in a down clue, letters go up).
'so' written backwards gives 'os'.
'a'+'prop'+'os'='APROPOS'
'is' acts as a link.
(Other definitions for apropos that I've seen before include "To the point about poor spa" , "fitting" , "With reference to, concerning" , "Appropriate" , "Regarding" .)