So very good it's disconcerting? (8)
I believe the answer is:
rattling
'disconcerting?' is the definition.
(rattle can mean to unnerve or disconcert)
'so very good it's' is the wordplay.
I cannot quite see how this works, but
'good' could be 'g' (abbreviation) and 'g' is present in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
The remaining letters 'ratlin' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for rattling that I've seen before include "Smart" , "Travelling at great speed" , "Noisily shaking (as in good yarn)" , "Quick and energetic" , "worrying" .)