Dive practice on vacation includes breathing apparatus (6)
I believe the answer is:
plunge
'dive' is the definition.
(synonyms)
'practice on vacation includes breathing apparatus' is the wordplay.
'on vacation' means to remove the middle letters.
'includes' means one lot of letters goes inside another.
'breathing apparatus' becomes 'lung' (I've seen this before).
'practice' with its centre removed is 'pe'.
'pe' enclosing 'lung' is 'PLUNGE'.
(Other definitions for plunge that I've seen before include "Pitch" , "Dive - submerge" , "Thrust or dive into" , "Sudden drop" , "Take the . . . . . . - jump right in" .)