Snog someone from Bangkok, you say — perhaps undoing this? (7)
I believe the answer is:
necktie
'perhaps undoing this?' is the definition.
(a necktie is worn around the neck)
'snog someone from bangkok you say' is the wordplay.
'snog someone' becomes 'neck' (I can't justify this - if you can you should give a lot more credence to this answer).
'from bangkok' becomes 'Thai' (Bangkok is in Thailand).
'you say' shows a homophone (sound like).
'thai' sounds like 'tie'.
'neck'+'tie'='NECKTIE'
(Other definitions for necktie that I've seen before include "Item of (American) clothing" , "Material worn knotted round throat" , "highly informal sort of party" , "Fashion accessory - cite Ken (anag)" , "Formal wear" .)