A niece regularly eating fish — and mushroom for example (6)
I believe the answer is:
agaric
'mushroom for example' is the definition.
(mushroom is a kind of agaric)
'a niece regularly eating fish' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'eating' indicates putting letters inside.
'fish' becomes 'gar' (gar is a kind of fish**).
'a'+'niece'='aniece'
The alternate letters of 'aniece' are 'aic'.
'aic' placed around 'gar' is 'AGARIC'.
'and' is the link.
(Other definitions for agaric that I've seen before include "Type of mushroom" , "fungal growth" , "Fungus of mushroom family" , "Fungus, eg, mushroom" .)