Falling into it is bad (5)
I believe the answer is:
decay
This clue could be a double definition.
'falling' is the first definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'it is bad' is the second definition.
The definition suggests a singular noun which matches the answer.
'into' acts as a link.
Can you help me to learn more?
(Other definitions for decay that I've seen before include "Rot, perish" , "Atrophy" , "Decline, decompose" , "Decompose or fall into ruin" , "Briefly fashionable" .)