Mythical beast -- gosh! -- inside a French home (7)
I believe the answer is:
unicorn
'mythical beast' is the definition.
(I've seen this before)
'gosh inside a french home' is the wordplay.
'gosh' becomes 'cor' (I've seen this before).
'inside' is an insertion indicator.
'a french' becomes 'un' ('a' in French).
'home' becomes 'in' ('he's home' can mean 'he's in').
'un'+'in'='unin'
'cor' going within 'unin' is 'UNICORN'.
(Other definitions for unicorn that I've seen before include "Lion's traditional adversary" , "Fabulous beast like horned horse" , "Fabled horned beast" , "Imaginary animal" , "Mythical white horse with one horn" .)