Last bit in signed document, thorough (8)
I believe the answer is:
detailed
'thorough' is the definition.
(I've seen this in another clue)
'last bit in signed document' is the wordplay.
'last bit' becomes 'tail' (the tail end of something).
'in' means one lot of letters goes inside another.
'signed document' becomes 'deed' (legal document).
'tail' put within 'deed' is 'DETAILED'.
(Other definitions for detailed that I've seen before include "Showing attention to fine features" , "Minutely described" , "Particular" , "Giving full particulars" , "Exhaustive" .)