Around 50 began to be alarmed (8)
I believe the answer is:
startled
'be alarmed' is the definition.
(thesaurus)
'around 50 began' is the wordplay.
'around' indicates putting letters inside.
'50' becomes 'l' (roman numeral).
'began' becomes 'started' ('start' can be a synonym of 'begin').
'l' inserted inside 'started' is 'STARTLED'.
'to' is the link.
(Other definitions for startled that I've seen before include "Frightened or suprised suddenly" , "Caused sudden alarm" , "Stunned, shocked" , "Surprised with fright" , "Taken aback" .)