Certainly not included in population of Earth (2,4)
I believe the answer is:
no fear
'certainly not' is the definition.
(phrase used to express a denial or refusal)
'included in population of earth' is the wordplay.
'included in' indicates a hidden word.
'NO FEAR' is hidden within 'population of earth'.
(Other definitions for no fear that I've seen before include "Certainly not (informal)" , "Informally, not likely to do - who's afraid?" , "Informally, certainly not., who's afraid" , "What brave man displays" , "What 7 [INTREPID] person will have" .)