Slam upstanding saint, causing gasps (5)
I believe the answer is:
pants
'gasps' is the definition.
('pant' can be a synonym of 'gasp')
'slam upstanding saint' is the wordplay.
'slam' becomes 'pan' (I've seen this in another clue).
'upstanding' shows that the letters should be reversed in order.
'saint' becomes 'st' (abbreviation).
'st' backwards is 'ts'.
'pan'+'ts'='PANTS'
'causing' is the link.
(Other definitions for pants that I've seen before include "See 17" , "What you get from a hot dog" , "Knickers" , "Frightful" , "longs" .)