Tremendous falling short in course of affair (10)
I believe the answer is:
thundering
'tremendous' is the definition.
(I've seen this before)
'falling short in course of affair' is the wordplay.
'falling short' becomes 'under'.
'in' is an insertion indicator.
'course of affair' becomes 'thing' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'under' put within 'thing' is 'THUNDERING'.
(Other definitions for thundering that I've seen before include "Making a loud noise" , "Tremendous" .)