Mourn mental decline (6)
I believe the answer is:
lament
'mourn' is the definition.
(I've seen this before)
'mental decline' is the wordplay.
'mental' can be anagrammed to 'LAMENT'.
Though, I am not clear how the anagram is indicated.
Can you help me to learn more?
(Other definitions for lament that I've seen before include "Mourn for; deplore" , "Express sorrow" , "complaint" , "Mourn, express regret" , "Song expressing grief" .)