Animal in a cell — a second has been regularly raised (6)
I believe the answer is:
amoeba
'animal in a cell' is the definition.
'a second has been regularly raised' is the wordplay.
'second' becomes 'mo' (both are short periods of time**).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'raised' says the letters should be written in reverse.
'has'+'been'='hasbeen'
The alternate letters of 'hasbeen' are 'abe'.
'abe' backwards is 'eba'.
'a'+'mo'+'eba'='AMOEBA'
(Other definitions for amoeba that I've seen before include "Unicellular form" , "One cell" , "Basic life form" , "Protozoan life-form" , "Microscopic animal found in water" .)