Make invalid endlessly fill unwieldy bags up (7)
I believe the answer is:
nullify
'make invalid' is the definition.
(I've seen this before)
'endlessly fill unwieldy bags up' is the wordplay.
'bags' indicates a hidden word.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'yfillun' is hidden within 'endlessly fill unwieldy'.
'yfillun' backwards is 'NULLIFY'.
(Other definitions for nullify that I've seen before include "Bring to naught" , "Neutralise" , "Cancel, abolish" , "Declare invalid" , "quash" .)