Sad Lyle keeps on being forsaken (6)
I believe the answer is:
lonely
'being forsaken' is the definition.
(thesaurus)
'sad lyle keeps on' is the wordplay.
'sad' is an anagram indicator.
'keeps' is an insertion indicator.
'lyle' is an anagram of 'lely'.
'lely' going around 'on' is 'LONELY'.
(Other definitions for lonely that I've seen before include "Isolated" , "... in need of company ..." , "Left" , "From the beginning (Latin)" , "In need of companionship" .)