Rolling ball carrying source of ink for pen (4)
I believe the answer is:
biro
'pen' is the definition.
(Biro is a make of pen)
'rolling ball carrying source of ink' is the wordplay.
'rolling' is a reversal indicator.
'ball' becomes 'orb' (both can mean a sphere).
'carrying' is an insertion indicator.
'source of' says to take the initial letters (the start - or source - of the word).
The first letter of 'ink' is 'i'.
'orb' going around 'i' is 'orib'.
'orib' written backwards gives 'BIRO'.
'for' is the link.
(Other definitions for biro that I've seen before include "Writing implement" , "Lazlo -, ballpoint pen inventor" , "Pocket item" , "Laszlo Jozsef -, ballpoint pen inventor" , "Type of ballpoint pen" .)