Feeling terribly intense over time (8)
I believe the answer is:
sentient
'feeling' is the definition.
('sentient' can be a synonym of 'feeling')
'terribly intense over time' is the wordplay.
'terribly' is an anagram indicator.
'over' is an insertion indicator.
'time' becomes 't'.
'intense' with letters rearranged gives 'senient'.
'senient' placed around 't' is 'SENTIENT'.
(Other definitions for sentient that I've seen before include "Capable of feeling things as in tenets" , "Conscious - aware" , "Able to perceive/feel" , "Endowed with feeling and consciousness" , "Having feelings" .)