A blog comment, say, wrecked defection (8)
I believe the answer is:
apostasy
'defection' is the definition.
(synonyms)
'a blog comment say wrecked' is the wordplay.
'blog comment' becomes 'post' (I've seen this in another clue).
'wrecked' is an anagram indicator.
'say' is an anagram of 'asy'.
'a'+'post'+'asy'='APOSTASY'
(Other definitions for apostasy that I've seen before include "Rejection of religion" , "Desertion" , "defection" , "Abandonment of religious faith" , "Abandonment of religious beliefs" .)