Can’t say for sure since entering doubtful, accomplished finally (15)

I believe the answer is:

unascertainable

I'm a little stuck... Click here to teach me more about this clue!

'can't say for sure' is the definition.
I can't tell whether this defines the answer.

'since entering doubtful accomplished finally' is the wordplay.
'since' becomes 'as' (I've seen this in other clues).
'entering' is an insertion indicator.
'doubtful' becomes 'uncertain' (similar in meaning).
'accomplished finally' becomes 'able' (I've seen this before. I am not sure about the 'finally' bit.).
'as' put inside 'uncertain' is 'unascertain'.
'unascertain'+'able'='UNASCERTAINABLE'

Can you help me to learn more?

I've seen this clue in The Hindu.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.