Best 1500m runner preserves energy (5)
I believe the answer is:
cream
'best' is the definition.
(thesaurus)
'1500m runner preserves energy' is the wordplay.
'1500m runner' becomes 'cram' (athlete Steve Cram).
'preserves' is an insertion indicator.
'energy' becomes 'e' (physics symbol as in E=mc2).
'cram' going around 'e' is 'CREAM'.
(Other definitions for cream that I've seen before include "Best part of the milk" , "Thick milk" , "treat for cat?" , "Elite; off-white" , "Dairy product" .)