Mistake made by troublesome child forgetting time (5)
I believe the answer is:
error
'mistake' is the definition.
(I know that mistake can be written as error)
'troublesome child forgetting time' is the wordplay.
'troublesome child' becomes 'terror' (I've seen this before).
'forgetting' indicates named letters should be taken away.
'time' becomes 't' (abbreviation).
'terror' with 't' taken away is 'ERROR'.
'made by' acts as a link.
(Other definitions for error that I've seen before include "Solecism" , "Faux pas" , "Fallacy, mistake" , "Way of operating" , "wrong" .)