Gymnast (7)
'gymnast' is the definition.
(acrobat is a kind of gymnast)
This is all the clue.
(Other definitions for acrobat that I've seen before include "Agile entertainer" , "Boat car (anag.)" , "Gymnastic entertainer" , "Tightrope-walker" , "Circus athlete" .)
(Other definitions for tumbler that I've seen before include "spilling its contents?" , "He performs feats" , "A glass; part of lock" , "Machine for drying" , "Drinking glass or old acrobat" .)