It's hard to keep fish in a dirty place (6)
I believe the answer is:
steely
'it's hard' is the definition.
(similar in meaning)
'to keep fish in a dirty place' is the wordplay.
'to keep' indicates putting letters inside.
'fish in' becomes 'eel' (eel is a type of fish. I am not sure about the 'in' bit.).
'a dirty place' becomes 'sty' (as in a pig sty).
'eel' going within 'sty' is 'STEELY'.
Can you help me to learn more?
(Other definitions for steely that I've seen before include "Strong" , "Inflexibly severe" , "Hard and metallic" , "Set off" , "firm" .)