Principals in ghastly armed robbery lost youthful looks in the lock-up (7)

'the lock-up' is the definition.
I can't judge whether this definition defines the answer.

'principals in ghastly armed robbery lost youthful looks' is the wordplay.
'principals in' says to take the initial letters.
'lost youthful looks' becomes 'aged' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The first letters of 'ghastly armed robbery' is 'gar'.
'gar'+'aged'='GARAGED'

'in' is the link.

Can you help me to learn more?