Declining, not fine, sick (6)
I believe the answer is:
ailing
'declining not fine sick' is the definition.
'ailing' can be an answer for 'sick' (I've seen this before). I am not sure about the 'declining not fine' bit.
'fine sick' is the wordplay.
'fine' becomes 'ai' (resembles Al, informal term for top quality).
'sick' becomes 'ling' (I am not sure about this - if you are sure you should believe this answer much more).
'ai'+'ling'='AILING'
Can you help me to learn more?
(Other definitions for ailing that I've seen before include "Ill" , "Troubled in mind or body" , "Infertile" , "Unwell, in poor condition" , "not doing very well" .)