Soft copper retiring (5)
I believe the answer is:
cushy
'soft' is the definition.
(synonyms)
'copper retiring' is the wordplay.
'copper' becomes 'cu' (Cu is the chemical symbol for copper).
'retiring' becomes 'shy' (both can mean reserved or introverted).
'cu'+'shy'='CUSHY'
(Other definitions for cushy that I've seen before include "16 21 [INDIAN IMPORT]" , "Undemanding, secure" , "Comfortable" , "Jammy" , "Simple" .)