Alarmed, began to throttle first of louts (8)
I believe the answer is:
startled
'alarmed' is the definition.
(I've seen this before)
'began to throttle first of louts' is the wordplay.
'began' becomes 'started' ('start' can be a synonym of 'begin').
'to throttle' means one lot of letters goes inside another (throttle can mean to strangle or smother).
'first of louts' becomes 'l' (1st letter of 'louts').
'started' placed around 'l' is 'STARTLED'.
(Other definitions for startled that I've seen before include "Caused sudden alarm" , "Affrighted" , "Frightened or suprised suddenly" , "Shocked, alarmed" , "Taken by surprise" .)