Developing rifts, initially (5)
I believe the answer is:
'initially' is the definition.
(I've seen this in another clue)
'developing rifts' is the wordplay.
'developing' is an anagram indicator (letters developed into a new form).
'rifts' with letters rearranged gives 'FIRST'.
(Other definitions for first that I've seen before include "Leading" , "Preceding all others" , "The one that precedes all others, in time say" , "Most important" , "See 1" .)