Sustenance for worried nit nurse? About time! (9)
I believe the answer is:
nutrients
'sustenance' is the definition.
(both can mean nourishment)
'worried nit nurse? about time' is the wordplay.
'worried' is an anagram indicator.
'about' indicates putting letters inside.
'time' becomes 't' (abbreviation).
'nit'+'nurse'='nitnurse'
'nitnurse' anagrammed gives 'nurients'.
'nurients' enclosing 't' is 'NUTRIENTS'.
'for' acts as a link.
(Other definitions for nutrients that I've seen before include "good things to digest" , "Foodstuffs" , "Some of life's essentials" .)