Force second lock (6)
I believe the answer is:
stress
'force' is the definition.
(stress is a kind of force)
'second lock' is the wordplay.
'second' becomes 's' (common abbreviation).
'lock' becomes 'tress' (I've seen this before).
's'+'tress'='STRESS'
(Other definitions for stress that I've seen before include "Physical or mental over-exertion" , "Tension, pressure" , "possible excuse for sickie?" , "Feeling of pressure" , "Underline - tension" .)