Returning parliament holds power but it's weak (5)
I believe the answer is:
tepid
'it's weak' is the definition.
(thesaurus)
'returning parliament holds power' is the wordplay.
'returning' says the letters should be written in reverse.
'parliament' becomes 'diet' (diet can mean a parliament in some countries).
'holds' indicates putting letters inside.
'power' becomes 'p' (symbol in physics).
'diet' going around 'p' is 'dipet'.
'dipet' reversed gives 'TEPID'.
'but' is the link.
(Other definitions for tepid that I've seen before include "Half-hearted" , "Moderately warm" , "without enthusiasm" , "lacking enthusiasm" , "Showing little interest or enthusiasm" .)