"Is that so?” (6)
'is that so?' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for really that I've seen before include "without a doubt" , "surely not?" , "Go on" , "In truth - one may be surprised to say it" , "In genuine way" .)
'is that so?' is the definition.
(I've seen this in another clue)
This is all the clue.