John's energy unrestricted (5)
I believe the answer is:
loose
'unrestricted' is the definition.
(I've seen this before)
'john's energy' is the wordplay.
'john's' becomes 'loos' (loo is a kind of john).
'energy' becomes 'e' (physics symbol as in E=mc2).
'loos'+'e'='LOOSE'
(Other definitions for loose that I've seen before include "Not restrained or attached" , "Free or immoral" , "Vague -- insecure" , "Untie(d)" , "Not crammed" .)