Appals a lot of cavalry - I fire wildly, breaking through (9)

'appals' is the definition.
('horrify' can be a synonym of 'appal')

'a lot of cavalry i fire wildly breaking through' is the wordplay.
'a lot of' means to remove the last letter (most of the word but not all of it).
'cavalry' becomes 'horse' ('horse cavalry' can be a synonym of 'cavalry').
'wildly' indicates an anagram.
'breaking through' indicates putting letters inside.
'horse' with its final letter removed is 'hors'.
'i'+'fire'='ifire'
'ifire' with letters rearranged gives 'rifie'.
'hors' placed around 'rifie' is 'HORRIFIES'.