Incredible temperature detected in a measurement at sea (10)
I believe the answer is:
astounding
'incredible' is the definition.
('astounding' can be similar in meaning to 'incredible')
'temperature detected in a measurement at sea' is the wordplay.
'temperature' becomes 't' (abbreviation).
'detected in' is an insertion indicator.
'measurement at sea' becomes 'sounding'.
'a'+'sounding'='asounding'
't' inserted within 'asounding' is 'ASTOUNDING'.
(Other definitions for astounding that I've seen before include "Amazing" , "Stunning" , "Extraordinary" .)