A niece regularly eating fish — and mushroom for example (6)
I believe the answer is:
Here is my best explanation:
'mushroom for example' is the definition.
(mushroom is a kind of agaric)
'a niece regularly eating fish' is the subsidiary indication.
'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**).
The alternating letters of 'aniece' are 'aic'.
'aic' enclosing 'gar' is 'agaric'.
'and' is the link.
(Other definitions for agaric that I've seen before include "Type of mushroom", "Mushroom", "fungus", "Fungus of mushroom family", "fungal growth" and "Fungus, eg, mushroom".)