Immense grip by snake's tail (4)
I believe the answer is:
huge
'immense' is the definition.
('huge' can be a synonym of 'immense')
'grip by snake's tail' is the wordplay.
'grip' becomes 'hug' (hug is a kind of grip).
'by' means one lot of letters go next to another.
'tail' suggests the final letters.
The last letter of 'snake' is 'e'.
'hug'+'e'='HUGE'
(Other definitions for huge that I've seen before include "Gigantic" , "Enormous" , "Unusually large" , "Jumbo" , "giant" .)