Express regret for acting mental (6)
I believe the answer is:
lament
'express regret' is the definition.
(I've seen this before)
'acting mental' is the wordplay.
'acting' indicates an anagram (letters become active or change).
'mental' with letters rearranged gives 'LAMENT'.
'for' is the link.
(Other definitions for lament that I've seen before include "Mourn, express regret" , "Express grief" , "Mourn (a person's death)" , "Mourning song or poem" , "requiem" .)