It's hard to keep fish in a dirty place (6)

I believe the answer is:

steely

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'it's hard' is the definition.
(similar in meaning)

'to keep fish in a dirty place' is the wordplay.
'to keep' is an insertion indicator.
'fish in' becomes 'eel' (eel is a kind of fish. I am not sure about the 'in' bit.).
'a dirty place' becomes 'sty' (I've seen this in another clue).
'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" .)

I've seen this clue in the Evening Standard.

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