Comfortable copper is retiring (5)
I believe the answer is:
cushy
'comfortable' is the definition.
(I've seen this before)
'copper is retiring' is the wordplay.
'copper' becomes 'cu' (Cu is the chemical symbol for copper).
'is retiring' becomes 'shy' (both can mean reserved or introverted).
'cu'+'shy'='CUSHY'
(Other definitions for cushy that I've seen before include "Undemanding (slang)" , "Easy, soft (colloq.)" , "Jammy" , "Not demanding" , "16 21 [INDIAN IMPORT]" .)