Bungling tail captures spy concealing note that's irrelevant (10)
I believe the answer is:
tangential
'irrelevant' is the definition.
(similar in meaning)
'bungling tail captures spy concealing note' is the wordplay.
I cannot quite see how this works, but
an anagram of 'tail' is 'tial' which is located in the answer.
'note' could be 'n' (abbreviation) and 'n' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'that's' acts as a link.
(Other definitions for tangential that I've seen before include "Irrelevant" , "Peripheral" .)