Set member up (3)
I believe the answer is:
gel
'set' is the definition.
(I've seen this before)
'member up' is the wordplay.
'member' becomes 'leg' (I've seen this before).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'leg' reversed gives 'GEL'.
(Other definitions for gel that I've seen before include "Leg it back to get some for your hair" , "Thicken" , "Come together, take shape (colloq.)" , "Semi-solid" , "Set, become firmer" .)