I'd entered, in a manner of speaking — but this wasn't intentional (8)
I believe the answer is:
accident
'this wasn't intentional' is the definition.
The definition suggests a singular noun which matches the answer.
'i'd entered in a manner of speaking' is the wordplay.
'entered in' is an insertion indicator.
'a manner of speaking' becomes 'accent' (accent is a kind of manner of speaking).
'id' placed within 'accent' is 'ACCIDENT'.
'but' acts as a link.
Can you help me to learn more?
(Other definitions for accident that I've seen before include "disaster" , "Unexpected and unplanned event" , "Collision" , "Prang" , "Fortune" .)