In theory, one should put this on the second lock (6)
I believe the answer is:
stress
'in theory one should put this' is the definition.
The definition suggests a singular noun which matches the answer.
'on the second lock' is the wordplay.
'on' is a charade indicator (letters next to each other).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'second' becomes 's' (common abbreviation).
'lock' becomes 'ress' (I've seen this in another clue).
't' put after 's' is 'st'.
'st'+'ress'='STRESS'
Can you help me to learn more?
(Other definitions for stress that I've seen before include "'Worry, pressure (6)'" , "Tension - accent" , "State mental or emotional strain" , "possible excuse for sickie?" , "Dwell on" .)