Emphasise needing second lock (6)
I believe the answer is:
stress
'emphasise' is the definition.
(I've seen this before)
'second lock' is the wordplay.
'second' becomes 's' (common abbreviation).
'lock' becomes 'tress' (I've seen this before).
's'+'tress'='STRESS'
'needing' acts as a link.
(Other definitions for stress that I've seen before include "Physical or mental over-exertion" , "Emphasis: result often of being under pressure" , "possible cause of fracture?" , "State mental or emotional strain" , "Highlight" .)