To thieve, we hear, is something hard (5)
I believe the answer is:
steel
'hard' is the definition.
'to thieve we hear' is the wordplay.
'to thieve' becomes 'steal' ('steal' can be a synonym of 'thieve').
'we hear' indicates a 'sounds like' (homophone) clue.
'steal' is a homophone of 'STEEL'.
'is something' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for steel that I've seen before include "Strength, determination" , "May be cold" , "Mixture of carbon and iron" , "Hard, strong, grey alloy" , "Hard iron alloy" .)