Jerk not thinking to rise (3)
I believe the answer is:
tug
'jerk' is the definition.
(I know that jerk can be written as tug)
'not thinking to rise' is the wordplay.
'not thinking' becomes 'gut' (as in a gut feeling).
'to rise' says the letters should be written in reverse.
'gut' back-to-front is 'TUG'.
(Other definitions for tug that I've seen before include "Small boat for towing others" , "Small boat designed to pull larger ships" , "Boat to pull larger one" , "Pull a form of boat" , "jerk" .)