Pudding rodent swallows slowly at first (6)
I believe the answer is:
mousse
'pudding' is the definition.
(I have seen 'Fluffy pudding' mean 'mousse' so perhaps 'pudding' could also mean 'mousse')
'rodent swallows slowly at first' is the wordplay.
'rodent' becomes 'mouse' (mouse is a kind of rodent).
'swallows' indicates putting letters inside.
'at first' suggests taking the first letters.
The first letter of 'slowly' is 's'.
'mouse' enclosing 's' is 'MOUSSE'.
(Other definitions for mousse that I've seen before include "Gel" , "For us, some kind of dessert" , "hair product" , "Foamy dish" , "Foamy hair preparation" .)