Bad lad with mare that's frightened (7)
I believe the answer is:
alarmed
'that's frightened' is the definition.
(alarming is a kind of frightening)
'bad lad with mare' is the wordplay.
'bad' indicates anagramming the letters.
'with' means one lot of letters go next to another.
'lad' put after 'mare' is 'marelad'.
'marelad' with letters rearranged gives 'ALARMED'.
(Other definitions for alarmed that I've seen before include "Experiencing a sudden sense of danger" , "Worried" , "Fear-stricken" , "Flapping" , "Apprehensive" .)