Freaks of nature: mongrels ain't regularly included (7)
I believe the answer is:
mutants
'freaks of nature' is the definition.
'mutants' can be an answer for 'freaks' (I've seen this before). I am not sure about the 'of nature' bit.
'mongrels ain't regularly included' is the wordplay.
'mongrels' becomes 'mutts' ('mutt' can be a synonym of 'mongrel').
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'included' is an insertion indicator.
The alternating letters of 'aint' are 'an'.
'mutts' placed around 'an' is 'MUTANTS'.
Can you help me to learn more?
(Other definitions for mutants that I've seen before include "freaks" , "Organisms with gene alterations" , "organism's genetic variations" .)