Female creature drover regularly observed (3)
I believe the answer is:
doe
'female creature' is the definition.
(female deer)
'drover regularly observed' is the wordplay.
'regularly observed' indicates alternate letters (letters selected at regular intervals).
The alternate letters of 'drover' are 'DOE'.
(Other definitions for doe that I've seen before include "Ode to a female deer" , "She's a deer" , "Female kangaroo" , "partner for a buck" , "Mummy rabbit" .)