Phenomenal temperature found in a measurement at sea (10)
I believe the answer is:
astounding
'phenomenal' is the definition.
(thesaurus)
'temperature found in a measurement at sea' is the wordplay.
'temperature' becomes 't' (abbreviation).
'found in' means one lot of letters goes inside another.
'measurement at sea' becomes 'sounding'.
'a'+'sounding'='asounding'
't' going into 'asounding' is 'ASTOUNDING'.
(Other definitions for astounding that I've seen before include "Stunning" , "Extraordinary" , "Amazing" .)