Prior to calculations, regularly spooks animals (7)
I believe the answer is:
possums
'animals' is the definition.
(possum is a kind of animal)
'prior to calculations regularly spooks' is the wordplay.
'prior to' means one lot of letters go next to another.
'calculations' becomes 'sums' (I've seen this before).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'spooks' are 'pos'.
'sums' put after 'pos' is 'POSSUMS'.
(Other definitions for possums that I've seen before include "Furry Australian animals" , "Small furry Australian marsupials" , "Sums sop up for American animals" .)