Power supplied by piece of legislation and treaty (4)
I believe the answer is:
pact
'treaty' is the definition.
(I know that treaty can be written as pact)
'power supplied by piece of legislation' is the wordplay.
'power' becomes 'p' (symbol in physics).
'supplied by' is a charade indicator (letters next to each other).
'piece of legislation' becomes 'act' (eg an act of parliament).
'p'+'act'='PACT'
'and' is the link.
(Other definitions for pact that I've seen before include "deal" , "Settlement" , "International treaty" , "'Accord, treaty (4)'" , "Formal agreement between parties or states" .)