Stop a nice man showing alarm (5)
I believe the answer is:
panic
'alarm' is the definition.
(alarm can mean panic or worry)
'stop a nice man showing' is the wordplay.
'showing' says the answer is hidden in the clue (the answer is shown in the clue).
'PANIC' is hidden within 'stop a nice man'.
(Other definitions for panic that I've seen before include "Widespread fear" , "Fear out of sixteen ac" , "Lose the head, yield to fear" , "Sudden alarm" , "Great consternation" .)