Staggering time taken by a measurement of depth (10)
I believe the answer is:
astounding
'staggering' is the definition.
(thesaurus)
'time taken by a measurement of depth' is the wordplay.
'time' becomes 't'.
'taken by' indicates putting letters inside.
'measurement of depth' becomes 'sounding'.
'a'+'sounding'='asounding'
't' inserted inside 'asounding' is 'ASTOUNDING'.
(Other definitions for astounding that I've seen before include "Stunning" , "Amazing" , "Extraordinary" .)