Inside of jacket keeps hot amid great burst of energy (9)
I believe the answer is:
lightning
'burst of energy' is the definition.
'inside of jacket keeps hot amid great' is the wordplay.
'inside of jacket' becomes 'lining'.
'keeps' indicates putting letters inside.
'hot' becomes 'h'.
'amid' means one lot of letters goes inside another.
'great' becomes 'gt' (abbreviation used in place names).
'h' placed inside 'gt' is 'ght'.
'lining' placed around 'ght' is 'LIGHTNING'.
(Other definitions for lightning that I've seen before include "Natural discharge of electricity" , "Electrical discharge from cloud" , "Flash in the sky" , "Natural electrical discharge" , "Precaution against electrical storms" .)