The Queen in Great Britain is keeping primarily little animals (7)
I believe the answer is:
gerbils
'animals' is the definition.
(gerbil is a kind of animal)
'the queen in great britain is keeping primarily little' is the wordplay.
'the queen' becomes 'er' (Elizabeth Regina**).
'in' means one lot of letters goes inside another.
'great britain' becomes 'gb'.
'keeping' is an insertion indicator.
'primarily' says to take the initial letters.
The initial letter of 'little' is 'l'.
'er' placed within 'gb' is 'gerb'.
'is' enclosing 'l' is 'ils'.
'gerb'+'ils'='GERBILS'
(Other definitions for gerbils that I've seen before include "Children's pet rodents" , "Burrowing mouse-like rodents" .)