Alas, senile, no longer holding back solitude (10)
I believe the answer is:
loneliness
'solitude' is the definition.
(I've seen this before)
'alas senile no longer holding back' is the wordplay.
'holding' indicates a hidden word.
'back' shows that the letters should be reversed in order.
'ssenilenol' is hidden within 'alas senile no longer'.
'ssenilenol' written backwards gives 'LONELINESS'.
(Other definitions for loneliness that I've seen before include "Solitary feeling" , "Desolation" , "Feeling of isolation" , "Unwelcome solitude" .)