Bank official retaining energy (4)
I believe the answer is:
reef
'bank' is the definition.
(thesaurus)
'official retaining energy' is the wordplay.
'official' becomes 'ref' (match official in football, say).
'retaining' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'ref' placed around 'e' is 'REEF'.
(Other definitions for reef that I've seen before include "Hazard for ships" , "Main bank" , "offshore bank" , "Submerged ridge of rock or coral near the surface" , "Kind of knot (avoided by sailors?)" .)