Song, then have rest (3,4)
I believe the answer is:
lie down
'rest' is the definition.
(I've seen this before)
'song then have' is the wordplay.
'song' becomes 'lied' (lied is a kind of song**).
'then' says to put letters next to each other.
'have' becomes 'own' ('own' can be a synonym of 'have').
'lied'+'own'='LIE-DOWN'
(Other definitions for lie down that I've seen before include "Recline" , "Now idle (anag) -- short rest" , "Take to one's bed" , "Put your feet up" , "Relax" .)