Mathematician's rule about conserving energy (5)
I believe the answer is:
euler
'mathematician's' is the definition.
(I've seen this before)
'rule about conserving energy' is the wordplay.
'about' is an anagram indicator.
'conserving' is an insertion indicator.
'energy' becomes 'e' (physics symbol as in E=mc2).
'rule' is an anagram of 'eulr'.
'eulr' placed around 'e' is 'EULER'.
(Other definitions for euler that I've seen before include "Swiss mathematician; sounds like lubricator" , "Leonhard -----, founder of mathematical analysis" , "Leonhard -, Eighteenth century Swiss mathematician" , "Leonhard --, C18 Swiss mathematician" , "Swiss scientist" .)