Not the case notes lawyers on vacation should open (5)
I believe the answer is:
false
'not the case' is the definition.
(not the case or not the truth)
'notes lawyers on vacation should open' is the wordplay.
'notes' becomes 'FAE' (three musical notes).
'on vacation' says to hollow out the word (remove centre letters).
'should open' indicates putting letters inside (some letters open a gap in others).
'lawyers' with its centre removed is 'ls'.
'fae' placed around 'ls' is 'FALSE'.
(Other definitions for false that I've seen before include "Sham" , "Spurious, untrue" , "Untrue - treacherous - counterfeit" , "Mendacious" , "Not in accordance with fact" .)