Careless mistake leads to someone having overdose (8)
I believe the answer is:
slipshod
'careless' is the definition.
(similar in meaning)
'mistake leads to someone having overdose' is the wordplay.
'mistake' becomes 'slip' (a slip is a blunder or mistake).
'leads to' indicates taking the first letters.
'overdose' becomes 'o.d.' (synonyms).
The initial letters of 'someone having' is 'sh'.
'slip'+'sh'+'od'='SLIPSHOD'
(Other definitions for slipshod that I've seen before include "Lacking in care" , "Disorganised" , "Sloppy and haphazard" , "Inattentive" , "Carelessly done" .)