Rodent includes son in dessert (6)
I believe the answer is:
mousse
'dessert' is the definition.
(mousse is a kind of dessert)
'rodent includes son' is the wordplay.
'rodent' becomes 'mouse' (mouse is a kind of rodent).
'includes' means one lot of letters goes inside another.
'son' becomes 's' (genealogical abbreviation for son).
'mouse' going around 's' is 'MOUSSE'.
'in' acts as a link.
(Other definitions for mousse that I've seen before include "F" , "Whipped cream dessert" , "Foamy hair preparation" , "Whipped creamy dessert" , "Capital preparation" .)