Harassed following force when protected by rocky citadel (9)
I believe the answer is:
afflicted
'harassed' is the definition.
(harass can mean to afflict or trouble)
'following force when protected by rocky citadel' is the wordplay.
'following' becomes 'f.' (used when citing page numbers etc).
'force' becomes 'F' (symbol used in Physics e.g. f=ma).
'when protected by' means one lot of letters goes inside another (inserted letters are held or protected).
'rocky' indicates an anagram.
'f'+'f'='ff'
'citadel' with letters rearranged gives 'alicted'.
'ff' going inside 'alicted' is 'AFFLICTED'.
(Other definitions for afflicted that I've seen before include "Caused pain to" , "Troubled" , "Diseased; suffering" .)