Catch one's breath seeing end of long snake (4)
I believe the answer is:
gasp
'catch one's breath' is the definition.
(I've seen this before)
'end of long snake' is the wordplay.
'end of' says to take the final letters.
'snake' becomes 'asp' (asp is a kind of snake**).
The last letter of 'long' is 'g'.
'g'+'asp'='GASP'
'seeing' is the link.
(Other definitions for gasp that I've seen before include "Sudden intake of breath" , "Short sharp intake of breath" , "Convulsive breath" , "Breathe irregularly" , "Utter breathlessly" .)