Writer's old bone sent back (4)
I believe the answer is:
biro
'writer's' is the definition.
(I've seen this before)
'old bone sent back' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'bone' becomes 'rib' (rib is a type of bone).
'sent back' says the letters should be written in reverse.
'o'+'rib'='orib'
'orib' reversed gives 'BIRO'.
(Other definitions for biro that I've seen before include "Writing implement" , "Type of ballpoint pen" , "Laszlo Jozsef -, ballpoint pen inventor" , "Pocket item" , "writer" .)