Pressure drops over time, definitely (3,7)
I believe the answer is:
'definitely' is the definition.
(for certain means definitely or absolutely)
'pressure drops over time' is the wordplay.
I cannot really see how this works, but
'pressure' could be 'force' (synonyms) and 'force' is found within the answer.
'time' could be 't' (abbreviation) and 't' is located in the answer.
'drops' could be 'rain' (drops of water) and 'rain' is present in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
(Other definitions for for certain that I've seen before include "surely" , "without a doubt" , "Assuredly" , "definitely" .)