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 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'.
'seeing' is the link.
(Other definitions for gasp that I've seen before include "Inhale sharply with astonishment", "Utter breathlessly", "Gaps in struggle for breath", "show amazement", "Sudden intake of breath", "Say "ah"", "Short laboured intake of breath", "Breathe sharply in amazement, horror etc", "short intake of breath" and "Struggle for breath with open mouth".)