Show interest in Brussels retaining the old power (3,2)
I believe the answer is:
eye up
'show interest in' is the definition.
'brussels retaining the old power' is the wordplay.
'brussels' becomes 'EU' (European Parliament is based in Brussels).
'retaining' is an insertion indicator (inserted letters are held or retained).
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'power' becomes 'p' (symbol in physics).
'eu' going around 'ye' is 'eyeu'.
'eyeu'+'p'='EYE UP'
(Other definitions for eye up that I've seen before include "Look at (someone) appraisingly" , "study" , "Look at (someone) as a potential partner" , "Appraise" , "Eagerly watch" .)