Acknowledge song first? Have a rest! (3,4)
I believe the answer is:
lie down
'have a rest' is the definition.
(I've seen this before)
'acknowledge song first?' is the wordplay.
'acknowledge' becomes 'own' (I've seen this before).
'song' becomes 'lied' (lied is a kind of song**).
'first?' says to put letters next to each other.
'own' after 'lied' is 'LIE-DOWN'.
(Other definitions for lie down that I've seen before include "Relax" , "Assume recumbent position" , "Short rest on bed/sofa" , "Become 14" , "Put your feet up" .)