Surprised reaction when girl's first met snake (4)
I believe the answer is:
gasp
'surprised reaction' is the definition.
(I've seen this in another clue)
'girl's first met snake' is the wordplay.
'girl's first' becomes 'g' (ast letter of 'girl').
'met' says to put letters next to each other.
'snake' becomes 'asp' (asp is a kind of snake).
'g'+'asp'='GASP'
'when' acts as a link.
(Other definitions for gasp that I've seen before include "Noisily breathe" , "Gaps in struggle for breath" , "Convulsive breath" , "First Greek letter" , "Have trouble breathing" .)