With frequency an old don drifts into sleep (4,2,3)
I believe the answer is:
land of nod
'sleep' is the definition.
(thesaurus)
'with frequency an old don drifts' is the wordplay.
'with' is an insertion indicator.
'frequency' becomes 'f' (abbreviation e.g. FM radio).
'drifts' is an anagram indicator.
'an'+'old'+'don'='anolddon'
'anolddon' is an anagram of 'landonod'.
'f' put into 'landonod' is 'LAND OF NOD'.
'into' acts as a link.
(Other definitions for land of nod that I've seen before include "Where Cain went after slaying Abel" , "Where Cain fled to, east of Eden" , "Where a murderer took refuge" , "Where Cain went after killing Abel" , "Early dwelling place" .)