A support thus returning to the purpose (7)
I believe the answer is:
apropos
'to the purpose' is the definition.
(I've seen this before)
'a support thus returning' is the wordplay.
'support' becomes 'prop' (prop up can mean to support).
'thus' becomes 'so' (synonyms).
'returning' shows that the letters should be reversed in order.
'so' written backwards gives 'os'.
'a'+'prop'+'os'='APROPOS'
(Other definitions for apropos that I've seen before include "With reference to, concerning" , "With regard to" , "Concerned with" , "by the way" , "Regarding" .)