One copper perhaps gathers nobody's at home (2,4,7)
I believe the answer is:
in ones element
'at home' is the definition.
(in one's element means at home or comfortable)
'one copper perhaps gathers nobody's' is the wordplay.
'one' becomes 'i' (Roman numeral).
'copper perhaps' becomes 'element' (copper is a chemical element).
'gathers' is an insertion indicator.
'nobody's' becomes 'none's'.
'i'+'element'='ielement'
'ielement' going around 'nones' is 'IN ONES ELEMENT'.
(Other definitions for in ones element that I've seen before include "Within a pleasing environment" , "Occupied with something that gives great pleasure" , "Involved in naturally suited sphere" .)