Pressure finally breaks lock (6)
I believe the answer is:
stress
'pressure' is the definition.
(thesaurus)
'finally breaks lock' is the wordplay.
'finally' suggests the final letters.
'lock' becomes 'tress' (I've seen this before).
The final letter of 'breaks' is 's'.
's'+'tress'='STRESS'
(Other definitions for stress that I've seen before include "Underline - tension" , "Emphasise the pressure" , "Tension; emphasis" , "Tension, pressure" , "Put weight on" .)