Progressing sequentially (6)
I believe the answer is:
linear
'progressing sequentially' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for linear that I've seen before include "Long and narrow, of one dimension" , "In one dimension, I learn (anag.)" , "Consisting of lines" , "Straight; sequential" , "-- Motor; -- equation" .)