Witness jumpers being lifted (4)
I believe the answer is:
spot
'witness' is the definition.
(thesaurus)
'jumpers being lifted' is the wordplay.
'jumpers' becomes 'tops'.
'being lifted' is a reversal indicator (going up in down clue).
'tops' reversed gives 'SPOT'.
(Other definitions for spot that I've seen before include "Notice - fictional puppy" , "Sight - site" , "See; pimple" , "point on snooker table" , "See; location" .)