About a quarter recover? How surprising! (6)
I believe the answer is:
Here is my best explanation:
'how surprising' is the definition.
('really?' can be said in disbelief or shock)
'about a quarter recover?' is the wordplay.
'about' indicates putting letters inside.
'a quarter' becomes 'e'.
'recover?' becomes 'rally' (rallying is a kind of recovering).
'e' inserted within 'rally' is 'REALLY'.
(Other definitions for really that I've seen before include "Truly, actually", "as a matter of fact", "In truth - join forces again", "Honestly, it's a fact", "In genuine way", "Undoubtedly", "are you sure?", "Is that so?", "In actual fact" and "surprised expression".)