See enormous energy unrestrained (5)
I believe the answer is:
loose
'unrestrained' is the definition.
(I've seen this in another clue)
'see enormous energy' is the wordplay.
'see' becomes 'lo' (archaic exclamation meaning 'look!').
'enormous' becomes 'os' (I've seen this in other clues).
'energy' becomes 'e' (physics symbol as in E=mc2).
'lo'+'os'+'e'='LOOSE'
(Other definitions for loose that I've seen before include "Untie(d)" , "poor fit?" , "Showing fast behaviour" , "Not restrained or attached" , "Lax or at large" .)