A type of toast popular on the sides today (6)
I believe the answer is:
french
'a type of toast' is the definition.
'popular on the sides today' is the wordplay.
I cannot really see how this works, but
'on' could be 're' ('on' can be mean 'with reference to') and 're' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for french that I've seen before include "Glass door" , "A Romance language" , "From Paris possibly" , "Take this leave without permission" , "European language" .)