Hard quality that’s appropriate when speaking? (5)
I believe the answer is:
steel
'hard quality' is the definition.
(quality of being hard or tough)
'appropriate when speaking?' is the wordplay.
'appropriate' becomes 'steal' (I've seen this before).
'when speaking?' indicates a 'sounds like' (homophone) clue.
'steal' is a homophone of 'STEEL'.
'that's' acts as a link.
(Other definitions for steel that I've seen before include "Make hard and unfeeling" , "Strength, determination" , "Form of iron" , "May be cold" , "Iron/carbon alloy" .)