Shocked reaction up in The Alps again (4)
I believe the answer is:
gasp
'shocked reaction' is the definition.
(I've seen this in another clue)
'up in the alps again' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'in' says the answer is hidden in the clue.
'psag' is hidden within 'alps again'.
'psag' backwards is 'GASP'.
(Other definitions for gasp that I've seen before include "Have trouble breathing" , "Struggle for breath with open mouth" , "Breathe sharply in amazement, horror etc" , "First Greek letter" , "Say "ah"" .)