Rudely knocked up a knight to open safe (5,4,3,3)
I believe the answer is:
under lock and key
'safe' is the definition.
(I've seen this in another clue)
'rudely knocked up a knight to open' is the wordplay.
'up' is an anagram indicator (letters get chucked up).
'knight' becomes 'N' (chess abbreviation).
'to open' is an insertion indicator (some letters open a gap in others).
'a'+'n'='an'
'rudely'+'knocked'='rudelyknocked'
'rudelyknocked' with letters rearranged gives 'underlockdkey'.
'underlockdkey' going around 'an' is 'UNDER LOCK AND KEY'.
(Another definition for under lock and key that I've seen is " Imprisoned".)