Thorough but Ted, returning, wasn’t well (8)
I believe the answer is:
detailed
'thorough' is the definition.
(I've seen this in another clue)
'ted returning wasn't well' is the wordplay.
'returning' is a reversal indicator.
'wasn't well' becomes 'ailed' (I've seen this in another clue).
'ted' backwards is 'det'.
'det'+'ailed'='DETAILED'
'but' is the link.
(Other definitions for detailed that I've seen before include "Showing attention to fine features" , "Giving full particulars" , "Showing even small features" , "supplying full particulars" , "Itemized or intricate" .)