Mention work in energy (5)
I believe the answer is:
opine
'mention' is the definition.
(thesaurus)
'work in energy' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'energy' becomes 'e' (physics symbol as in E=mc2).
'op'+'in'+'e'='OPINE'
(Other definitions for opine that I've seen before include "Express a point of view" , "Give one's view" , "Express one's opinion openly" , "Proffer a view" , "Suppose" .)