Tolerate an offer on energy (5)
I believe the answer is:
abide
'tolerate' is the definition.
(synonyms)
'an offer on energy' is the wordplay.
'an' becomes 'a'.
'offer' becomes 'bid' (bid is a kind of offer).
'on' means one lot of letters go next to another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'a'+'bid'+'e'='ABIDE'
(Other definitions for abide that I've seen before include "Stand" , "Stomach" , "Hack" , "Endure, tolerate" , "'. . . . . with me, fast falls the eventide'" .)