Drive home last of nails on lock (6)
I believe the answer is:
stress
'drive home' is the definition.
(drive home can mean to stress or emphasise)
'last of nails on lock' is the wordplay.
'last of' suggests the final letters.
'on' says to put letters next to each other.
'lock' becomes 'tress' (both can mean a piece of hair).
The final letter of 'nails' is 's'.
's'+'tress'='STRESS'
(Other definitions for stress that I've seen before include "Emphasis on anxiety?" , "Tension - accent" , "'Worry, pressure (6)'" , "Force" , "Draw attention to" .)