Rifts developing initially (5)
I believe the answer is:
first
'initially' is the definition.
(thesaurus)
'rifts developing' is the wordplay.
'developing' is an anagram indicator.
'rifts' with letters rearranged gives 'FIRST'.
(Other definitions for first that I've seen before include "Earliest; good degree" , "in dominant position" , "In winning position" , "Most important" , "Initial" .)