Engine horn is or should be blown with energy (4,5)
I believe the answer is:
iron horse
'engine' is the definition.
(iron horse is a kind of engine)
'horn is or should be blown with energy' is the wordplay.
'should be blown with' indicates anagramming the letters (letters blown around).
'energy' becomes 'e' (physics symbol as in E=mc2).
'horn'+'is'+'or'='hornisor'
'hornisor' anagrammed with 'e' can make 'IRON HORSE'.
(Other definitions for iron horse that I've seen before include "Old name for locomotive" , "Train leader" , "worn-out engine" , "film once in America!" , "stronger than the one left at Troy?" .)