Both sides in turn quickly succumbing to very strong smell (5)
I believe the answer is:
whiff
'smell' is the definition.
(whiffing is a kind of smelling)
'both sides in turn quickly succumbing to very strong' is the wordplay.
I cannot quite understand how this works, but
'strong' could be 'ff' and 'ff' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for whiff that I've seen before include "Suspicion" , "Briefly-caught smell" , "Passing smell" , "Scent in the air?" , "Slight smell" .)