About to dive into water? Hold it! (6)
I believe the answer is:
breath
'hold it' is the definition.
(you can hold your breath)
'about to dive into water?' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'to dive into' indicates putting letters inside.
'water?' becomes 'bath' (bath water).
're' inserted into 'bath' is 'BREATH'.
(Other definitions for breath that I've seen before include "It is drawn all the time" , "Respiration" , "Air that's inhaled and exhaled" , "Slight movement of air" , "from which inspiration may be drawn" .)