Anteater moving around base of anthill every second (9)
I believe the answer is:
alternate
'every second' is the definition.
(synonyms)
'anteater moving around base of anthill' is the wordplay.
'moving' indicates an anagram.
'around' is an insertion indicator.
'base of' says to take the final letters.
The last letter of 'anthill' is 'l'.
'anteater' with letters rearranged gives 'aternate'.
'aternate' going around 'l' is 'ALTERNATE'.
(Other definitions for alternate that I've seen before include "Change to first the one, then the other" , "Take turns" , "Occur in turn repeatedly" , "deputy" , "Substitute" .)