Fourth guest in resort, taken aback to see snakes (4)
I believe the answer is:
asps
'snakes' is the definition.
(asp is a kind of snake)
'fourth guest in resort taken aback' is the wordplay.
'fourth guest' becomes 's' (4th letter of 'guest').
'in' is an insertion indicator.
'resort' becomes 'spa' (spa is a kind of resort).
'taken aback' says the letters should be written backwards.
's' inserted into 'spa' is 'spsa'.
'spsa' back-to-front is 'ASPS'.
'to see' is the link.
(Other definitions for asps that I've seen before include "Small vipers" , "Cleopatra's snakes" , "Cleopatra's cobras" , "Egyptian cobras" , "Poplars" .)