Rightly supporting evasive reply (8)
I believe the answer is:
properly
'rightly' is the definition.
(I've seen this before)
'supporting evasive reply' is the wordplay.
'supporting' becomes 'pro' (being pro something is supporting it).
'evasive' indicates anagramming the letters (letters evade or dodge their original positions).
'reply' anagrammed gives 'perly'.
'pro'+'perly'='PROPERLY'
(Other definitions for properly that I've seen before include "in appropriate fashion" , "In the right manner" , "In the right way" , "Suitably, correctly" , "in a suitable manner" .)