Rodent eats second hair product (6)
I believe the answer is:
mousse
'hair product' is the definition.
(I've seen this before)
'rodent eats second' is the wordplay.
'rodent' becomes 'mouse' (mouse is a kind of rodent).
'eats' means one lot of letters goes inside another.
'second' becomes 's' (common abbreviation).
'mouse' going around 's' is 'MOUSSE'.
(Other definitions for mousse that I've seen before include "hair product" , "Frothy dish - hairdressing" , "Whipped savoury dish" , "Pope's representative" , "Aerosol foam used in hair styling" .)