Imagine tot with energy (5)
I believe the answer is:
dream
'imagine' is the definition.
(dreaming is a kind of imagining)
'tot with energy' is the wordplay.
'tot' becomes 'dram' (I've seen this before).
'with' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'dram' going around 'e' is 'DREAM'.
(Other definitions for dream that I've seen before include "Fantasy; imagine" , "It's experienced during sleep" , "Aspiration" , "Sleeping thoughts" , "Distant ambition" .)