It can lead to error (5)
I believe the answer is:
trial
'error' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'it can lead' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'can lead' becomes 'rial' (I can't justify this - if you can you should give a lot more credence to this answer).
't'+'rial'='TRIAL'
'to' is the link.
Can you help me to learn more?
(Other definitions for trial that I've seen before include "Judicial hearing and judgment" , "Court appearance on charge - it's an ordeal" , "Suffering - litigation" , "Experiment - adversity" , "Ordeal or lawsuit" .)