Fail to develop (8)
I believe the answer is:
stagnate
'fail to develop' is the definition.
(stagnate can mean to fail to develop or go stale)
This is the entire clue.
(Other definitions for stagnate that I've seen before include "Lapse into inactivity" , "Cease to flow or move" , "Cease activity" , "Become dull through inactivity" , "Tan gates could vegetate" .)