Retiring copper's first to get comfortable (5)
I believe the answer is:
cushy
'comfortable' is the definition.
(thesaurus)
'retiring copper's first' is the wordplay.
'retiring' becomes 'shy' (I've seen this in other clues).
'copper' becomes 'cu' (chemical symbol).
'first' says to put letters next to each other.
'shy' after 'cu' is 'CUSHY'.
'to get' is the link.
(Other definitions for cushy that I've seen before include "Undemanding (slang)" , "Easy, soft (colloq.)" , "Comfortable" , "Jammy" , "Undemanding, secure" .)