Exert pressure on this last lock (6)
I believe the answer is:
stress
'exert pressure on this last lock' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'on this last lock' is the wordplay.
'on' is a charade indicator (letters next to each other).
'last' suggests the final letters.
'lock' becomes 'tress' (both can mean a piece of hair).
The last letter of 'this' is 's'.
's' put next to 'tress' is 'STRESS'.
Can you help me to learn more?
(Other definitions for stress that I've seen before include "Physical or mental over-exertion" , "why off work?" , "Pressure or strain" , "Particular emphasis" , "possible excuse for sickie?" .)