Double jeopardy has caught people almost entirely off guard, initially (12)
I believe the answer is:
doppelganger
'double' is the definition.
(I've seen this before)
'jeopardy has caught people almost entirely off guard initially' is the wordplay.
'jeopardy' becomes 'danger' (I've seen this before).
'has caught' is an insertion indicator.
'almost entirely' means to remove the last letter.
'off' indicates anagramming the letters.
'initially' says to take the initial letters.
The first letter of 'guard' is 'g'.
'people' with its last letter taken off is 'peopl'.
'peopl' anagrammed gives 'oppel'.
'oppel'+'g'='oppelg'
'danger' placed around 'oppelg' is 'DOPPELGANGER'.
(Other definitions for doppelganger that I've seen before include "A person's spitting image" , "Ghostly double of a living person - green Lapp god (anag)" , "Spirit double" , "Living person's double" , "Ghostly double that haunts its living counterpart" .)