Line in second working socket possibly moving suddenly (9,8)

'line in' is the definition.
The definition suggests an adjective which matches the answer.

'second working socket possibly moving suddenly' is the wordplay.
I cannot quite see how this works, but
'second' could be 'mo' (both are short periods of time) and 'mo' is found within the answer.
'working' could be 'on' (e.g. a machine is working or on) and 'on' is found within the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?