Penned in doggerel, one line's spreading sad feelings (10)
I believe the answer is:
loneliness
'sad feelings' is the definition.
(feelings of loneliness)
'penned in doggerel one line's spreading' is the wordplay.
'penned in' says the answer is hidden in the clue (some letters blocked or penned in).
'LONELINESS' is hidden in the letters of 'doggerel one lines spreading'.
(Other definitions for loneliness that I've seen before include "Feeling of isolation" , "Desolation" , "Unwelcome solitude" , "Solitary feeling" .)