One of two bound for power (4)
I believe the answer is:
prop
'one of two bound' is the definition.
(rugby)
'for power' is the wordplay.
'for' becomes 'pro' (as in being 'pro' something).
'power' becomes 'p' (symbol in physics).
'pro'+'p'='PROP'
(Other definitions for prop that I've seen before include "Object used on the set of a play" , "Support; object used on stage" , "Shore for 4 down" , "Set piece" , "Moveable object used on stage" .)