Red mullet, say, always being caught (8)
I believe the answer is:
feverish
'red' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I can't see?
'mullet say always being caught' is the wordplay.
'mullet say' becomes 'fish' (mullet is a kind of fish).
'always' becomes 'ever' (synonyms).
'being caught' indicates putting letters inside.
'fish' going around 'ever' is 'FEVERISH'.
Can you help me to learn more?
(Other definitions for feverish that I've seen before include "Febrile" , "Restless with excitement or agitation - got ague?" , "Suffering from a high temperature" , "hectic" , "restless state" .)