Serious blow from priest or magus (5)
I believe the answer is:
storm
'serious blow' is the definition.
'storm' can be an answer for 'blow' (I have seen 'Awful blow' mean 'storm' so perhaps 'blow' could also mean 'storm'). I'm not certain of the 'serious' bit.
'from priest or magus' is the wordplay.
'from' says the answer is hidden in the clue.
'STORM' is hidden within 'priest or magus'.
Can you help me to learn more?
(Other definitions for storm that I've seen before include "Force 10 blow" , "Destructive wind" , "Feature of weather" , "X-Men character" , "Big fuss" .)