Mammals in lair bitten by English terriers repeatedly (8)
I believe the answer is:
edentata
'mammals' is the definition.
(I've seen this before)
'lair bitten by english terriers repeatedly' is the wordplay.
'lair' becomes 'den' (synonyms).
'bitten by' means one lot of letters goes inside another (I've seen this in other clues).
'english' becomes 'E' (abbreviation).
'terriers' becomes 'ta' (nickname for Territorial Army members).
'repeatedly' indicates the doubling of some letters.
'ta' doubled is 'tata'.
'e'+'tata'='etata'
'den' placed within 'etata' is 'EDENTATA'.
'in' acts as a link.
(Other definitions for edentata that I've seen before include "there are various animals in it" , "Group of creatures" , "mammals" , "Beastly group" , "toothless animals" .)