How to save energy? Party 'n' dance! (7)
I believe the answer is:
hoedown
'dance' is the definition.
(I've seen this before)
'how to save energy? party n' is the wordplay.
'to save' means one lot of letters goes inside another.
'energy?' becomes 'e' (physics symbol as in E=mc2).
'party' becomes 'do' (do is a kind of party).
'e'+'do'='edo'
'how' placed around 'edo' is 'hoedow'.
'hoedow'+'n'='HOEDOWN'
(Other definitions for hoedown that I've seen before include "Woodhen (anag.)" , "US country dance" , "Boisterous American folk dance" , "American country dance" , "Barn-dance" .)