Leading American kept hoarding energy (5)
I believe the answer is:
ahead
'leading' is the definition.
(to lead is to go ahead)
'american kept hoarding energy' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'kept' becomes 'had' (I've seen this before).
'hoarding' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'had' enclosing 'e' is 'head'.
'a'+'head'='AHEAD'
(Other definitions for ahead that I've seen before include "In the future" , "at pole" , "In the leading position" , "In the vanguard" , "In front as in race" .)