Certainly not like Thomas (9)
I believe the answer is:
doubtless
'certainly' is the definition.
(I've seen this before)
'not like thomas' is the wordplay.
I cannot really see how this works, but
'not' could be 'less' (I've seen this in other clues) and 'less' is present in the answer.
The remaining letters 'doubt' 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).
Can you help me to learn more?
(Other definitions for doubtless that I've seen before include "of course" , "Probably, certainly" , "Positive" , "Unquestionably" , "Assuredly" .)