This last lock shows strain (6)
I believe the answer is:
stress
'strain' is the definition.
(synonyms)
'this last lock' is the wordplay.
'last' says to take the final letters.
'lock' becomes 'tress' (both can mean a piece of hair).
The final letter of 'this' is 's'.
's'+'tress'='STRESS'
'shows' is the link.
(Other definitions for stress that I've seen before include "nervous tension" , "Emphasise the strain" , "Ictus" , "Emotional tension" , "Force" .)