The other is heartened by it (5)
I believe the answer is:
aitch
'it' is the definition.
The definition suggests a singular noun which matches the answer.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for aitch that I've seen before include "It's dropped by Cockneys" , "that I will immediately succeed" , "what some drop?" , "Letter sound that's often dropped?" , "the start of hives?" .)