Worry as distinct period of change announced (4)
I believe the answer is:
faze
'worry' is the definition.
(faze can mean to worry or bother)
'distinct period of change announced' is the wordplay.
'distinct period of change' becomes 'phase' (a phase of development).
'announced' indicates a 'sounds like' (homophone) clue.
'phase' is a homophone of 'FAZE'.
'as' acts as a link.
(Other definitions for faze that I've seen before include "Disconcert; sounds like period" , "Shake" , "Unsettle - perturb" , "Throw from" , "Disturb, disconcert" .)