Elusive quality ultimately behind comfortable footwear (8)
I believe the answer is:
slippery
'elusive' is the definition.
(thesaurus)
'quality ultimately behind comfortable footwear' is the wordplay.
'ultimately' suggests the final letters.
'behind' is a charade indicator (letters next to each other).
'comfortable footwear' becomes 'slipper' (I've seen this before).
The last letter of 'quality' is 'y'.
'y' put after 'slipper' is 'SLIPPERY'.
(Other definitions for slippery that I've seen before include "Evasive, elusive" , "Hard to grasp, like an eel" , "Not to be trusted" , "found difficult to get hold of" , "Evasive like an eel" .)