Securing it, John turned key (5)
I believe the answer is:
vital
'key' is the definition.
(I know that key can be written as vital)
'securing it john turned' is the wordplay.
'securing' indicates putting letters inside.
'john' becomes 'lav' (both can mean a toilet).
'turned' is a reversal indicator.
'lav' backwards is 'val'.
'it' inserted within 'val' is 'VITAL'.
(Other definitions for vital that I've seen before include "Extremely important, essential to success" , "Characteristic of life" , "Critical, necessary" , "Absolutely necessary or life-giving" , "Crucially important, essential" .)