Strength of character (6)
'strength of character' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mettle that I've seen before include "Spirit could sound like iron, say" , "'Nerve, heart, courage (6)'" , "Strength of character, courage" , "The courage to carry on" , "Courage sounds like iron, for instance" .)
'strength of character' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for fibre that I've seen before include "Thread; stamina" , "Dietary material; thread" , "Threadlike substance" , "Texture - strength of character" , "Long fine thread" .)