I had backed ally, so shockingly traitorous (8)
I believe the answer is:
disloyal
'traitorous' is the definition.
('disloyal' can be similar in meaning to 'traitorous')
'i had backed ally so shockingly' is the wordplay.
'i had' becomes 'id' (I'd).
'backed' says the letters should be written in reverse.
'shockingly' indicates anagramming the letters.
'id' back-to-front is 'di'.
'ally'+'so'='allyso'
'allyso' anagrammed gives 'sloyal'.
'di'+'sloyal'='DISLOYAL'
(Other definitions for disloyal that I've seen before include "Unfaithful or unpatriotic" , "Faithless" , "Treasonable or treacherous" , "Deserting one's allegiance" , "Not faithful, treacherous" .)