Catch one's breath seeing end of long snake (4)
I believe the answer is:
Here is my best explanation:
'catch one's breath' is the definition.
(I've seen this before)
'end of long snake' is the subsidiary indication.
'end of' says to take the final letters.
'snake' becomes 'asp' (asp is a kind of snake**).
The final letter of 'long' is 'g'.
'seeing' is the link.
(Other definitions for gasp that I've seen before include "Inhale sharply with astonishment", "Convulsive breath", "Draw breath sharply", "Short laboured intake of breath", "Pant", "Sudden intake of breath", "Short sharp intake of breath", "Sharp intake of breath", "Sound of astonishment" and "Puff, struggle for breath".)