Mother protects one following rogue element (7)
I believe the answer is:
cadmium
'element' is the definition.
(cadmium is a kind of element)
'mother protects one following rogue' is the wordplay.
'mother' becomes 'mum' (mum is a kind of mother**).
'protects' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'following' means one lot of letters go next to another.
'rogue' becomes 'cad' (I've seen this before).
'mum' placed around 'i' is 'mium'.
'mium' after 'cad' is 'CADMIUM'.
(Other definitions for cadmium that I've seen before include "A yellow" , "Silvery metal, Cd" , "Metallic element, Cd" , "reactor component" , "Silvery-white metal, Cd" .)