What 'Arry dropped! (5)
I believe the answer is:
aitch
If this is the answer, I'm afraid I cannot explain it.
Can you help me to learn more?
(Other definitions for aitch that I've seen before include "that may be dropped" , "that I will immediately succeed" , "It's dropped by Cockneys" , "The letter h in full" , "first bit of hymn?" .)