Clearly form of tennis involving first of two players ultimately (9)
I believe the answer is:
Here is my best explanation:
'clearly' is the definition.
'form of tennis involving first of two players ultimately' is the wordplay.
'form of tennis' becomes 'doubles'.
'involving' means one lot of letters goes inside another.
'first of' indicates taking the first letters.
'ultimately' says to take the final letters.
The initial letter of 'two' is 't'.
The final letter of 'players' is 's'.
'doubles' going around 't' is 'doubtles'.
(Other definitions for doubtless that I've seen before include "Assuredly", "Definitely not", "Very probably", "Positive", "Probably, certainly", "Unquestionably", "of course" and "Certainly or very probably".)