A line and leads found tangled, delivering shock? (3,2,1,6)

I believe the answer is:

all of a sudden

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'delivering shock?' is the definition.
I can't tell whether this definition defines the answer.

'a line and leads found tangled' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'and' says to put letters next to each other.
'tangled' indicates an anagram.
'leads'+'found'='leadsfound'
'leadsfound' is an anagram of 'lofasudden'.
'a'+'l'+'lofasudden'='ALL OF A SUDDEN'

Can you help me to learn more?

(Other definitions for all of a sudden that I've seen before include "Without warning" , "Quickly and unexpectedly" , "In an unexpected manner" , "abruptly" .)

I've seen this clue in The Telegraph.

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