Fish has turned man - that's hard (6)
I believe the answer is:
enamel
'that's hard' is the definition.
I can't judge whether this defines the answer.
'fish has turned man' is the wordplay.
'fish' becomes 'eel' (eel is a type of fish).
'has' means one lot of letters goes inside another.
'turned' indicates an anagram.
'man' is an anagram of 'nam'.
'eel' placed around 'nam' is 'ENAMEL'.
Can you help me to learn more?
(Other definitions for enamel that I've seen before include "Hard, glossy protective coating" , "Hard shiny surface as on teeth" , "Decorative or protective covering on metal or glass." , "Hard white substance covering the crown of a tooth" , "Glassy substance" .)