He can have a startling effect on public opinion (8)
I believe the answer is:
alarmist
'he can have a startling effect on public opinion' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he can have a' is the wordplay.
I cannot really understand how this works, but
'he' could be 'al' and 'al' is found in the answer.
'a' is within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for alarmist that I've seen before include "Spreader of panic" , "One spreads unnecessary worry or fear" , "Creating needless worry" , "Like panicking passenger" , "flapping excessively" .)