Initial rifts developing (5)
I believe the answer is:
first
'initial' is the definition.
('first' can be similar in meaning to 'initial')
'rifts developing' is the wordplay.
'developing' indicates anagramming the letters.
'rifts' anagrammed gives 'FIRST'.
(Other definitions for first that I've seen before include "to begin with" , "at the outset" , "Rifts of supreme sort" , "in dominant position" , "Highest degree" .)