Shocked a girl, her arm struck, throbbing basically (6)
I believe the answer is:
aghast
'shocked' is the definition.
(synonyms)
'a girl her arm struck throbbing basically' is the wordplay.
'a girl' becomes 'ag' ().
'basically' suggests taking the first letters (the first letter could be its root or base).
The first letters of 'her arm struck throbbing' is 'hast'.
'ag'+'hast'='AGHAST'
(Other definitions for aghast that I've seen before include "Horror-struck" , "Very unhappy" , "Horrified, appalled" , "Stupefied with horrer" , "in awe" .)