Turn key if weeping outside - devastating (10)

'devastating' is the definition.
Although both the answer and definition are present participle verbs, I don't see how one could define the other.

'turn key if weeping outside' is the wordplay.
'turn key' becomes 'uc' (I can't explain this - if you can you should give a lot more credence to this answer).
'weeping' becomes 'crying' ('cry' can be a synonym of 'weep').
'outside' means one lot of letters goes inside another.
'uc'+'if'='ucif'
'ucif' placed into 'crying' is 'CRUCIFYING'.

Can you help me to learn more?