About a quarter recover? How surprising! (6)
I believe the answer is:
really
'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 "Dead" , "without a doubt" , "Truly, in fact" , "Undoubtedly" , "surely not?" .)