Details concerning nickel written up (4)
I believe the answer is:
info
'details' is the definition.
(short for information)
'concerning nickel written up' is the wordplay.
'concerning' becomes 'of'.
'nickel' becomes 'Ni' (Ni is the chemical symbol for nickel).
'written up' says the letters should be written in reverse.
'of'+'ni'='ofni'
'ofni' in reverse letter order is 'INFO'.
(Other definitions for info that I've seen before include "Intelligence (abbr)" , "Latest" , "Knowledge (colloq.)" , "counsel?" , "Dope" .)