Tentatively be one hundred per cent like Thomas? (10)

'tentatively' is the definition.
Both the answer and definition are adverbs. Perhaps there's a link between them I don't understand?

'one hundred per cent like thomas?' is the wordplay.
I cannot quite understand how this works, but
'like' could be 'ly' (similar in meaning) and 'ly' is located in the answer.
The remaining letters 'doubtful' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

'be' is the link.

Can you help me to learn more?