Rest of song now lost (3-4)
I believe the answer is:
lie down
'rest' is the definition.
(to lie down is to have a short rest)
'song now lost' is the wordplay.
'song' becomes 'lied' (type of German song).
'lost' is an anagram indicator.
'now' anagrammed gives 'own'.
'lied'+'own'='LIE-DOWN'
'of' is the link.
(Other definitions for lie down that I've seen before include "Rest (with or without sleep)" , "Take a rest (at length!)" , "Short rest on bed/sofa" , "joining rest" , "Relax" .)