Fixed enclosure containing fish (6)
I believe the answer is:
steely
'fixed enclosure containing fish' is the definition.
I can't tell whether this definition defines the answer.
'fixed enclosure containing fish' is the wordplay.
'fixed enclosure' becomes 'sty' (sty is a kind of enclosure. I am not sure about the 'fixed' bit.).
'containing' indicates putting letters inside.
'fish' becomes 'eel' (eel is a kind of fish).
'sty' enclosing 'eel' is 'STEELY'.
Can you help me to learn more?
(Other definitions for steely that I've seen before include "Strong" , "firm" , "Obdurate" , "Ruthless" , "Set off" .)