Ailing suspect is spied on by old copper (10)
I believe the answer is:
indisposed
'ailing' is the definition.
('indisposed' can be a synonym of 'ailing')
'suspect is spied on by old copper' is the wordplay.
'suspect' indicates an anagram.
'by' is a charade indicator (letters next to each other).
'old copper' becomes 'd'.
'is'+'spied'+'on'='isspiedon'
'isspiedon' is an anagram of 'indispose'.
'indispose'+'d'='INDISPOSED'
(Other definitions for indisposed that I've seen before include "Slightly unwell" , "Out of sorts" , "Poorly" .)