Hang rabbit up (3)
I believe the answer is:
sag
'hang' is the definition.
(thesaurus)
'rabbit up' is the wordplay.
'rabbit' becomes 'gas' (I've seen this before).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'gas' back-to-front is 'SAG'.
(Other definitions for sag that I've seen before include "Subsidence" , "Decline" , "Sink under weight" , "Sink (under load)" , "Drop" .)