Adult liable to finish early, overall (5)
I believe the answer is:
apron
'overall' is the definition.
'adult liable to finish early' is the wordplay.
'adult' becomes 'a' (abbreviation).
'liable' becomes 'prone' (both can mean having a tendency for something).
'to finish early' means to remove the last letter (I've seen 'finishing early' mean this).
'prone' with its final letter taken off is 'pron'.
'a'+'pron'='APRON'
(Other definitions for apron that I've seen before include "Domestic protection" , "stage perhaps" , "Protective kitchen wear" , "Airport area for manoeuvring and loading planes" , "Protective cover" .)