The mere suggestion of endlessly bad saints is fishy! (4)
I believe the answer is:
bass
'fishy' is the definition.
Both the definition and answer are adjectives. Perhaps there's a link between them I don't understand?
'the mere suggestion of endlessly bad saints' is the wordplay.
'the mere suggestion' becomes 'b' (I can't justify this - if you can you should give a lot more credence to this answer).
'of endlessly' says to take the centre (I've seen 'endlessly' mean this (the ends of the words are removed)).
'saints' becomes 'ss' (an abbreviation for saint written twice).
The middle letter of 'bad' is 'a'.
'b'+'a'+'ss'='BASS'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for bass that I've seen before include "Scottish rock" , "Lowest register of the male voice" , "Fish - low in pitch" , "Fish, guitar" , "Fish for the low-voiced" .)