On reflection, Houses of Parliament associated with low energy (5)
I believe the answer is:
oomph
'energy' is the definition.
(I've seen this before)
'on reflection houses of parliament associated with low' is the wordplay.
'on reflection' says the letters should be written in reverse.
'houses of parliament' becomes 'hp'.
'associated with' means one lot of letters go next to another.
'low' becomes 'moo' ('moo' can be a synonym of 'low').
'hp'+'moo'='hpmoo'
'hpmoo' reversed gives 'OOMPH'.
(Other definitions for oomph that I've seen before include "Exciting energy" , "Informal energy or enthusiasm, usually in a girl" , "Get-up-and-go" , "Sexy, exciting quality" , "Vitality - sex appeal (informal)" .)