Perhaps May’s second day, all very endless and black (5,4)
I believe the answer is:
modal verb
'perhaps may's' is the definition.
('may' is a modal verb)
'second day all very endless and black' is the wordplay.
'second day all' becomes 'modal' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'endless' means to remove the last letter.
'and' is a charade indicator (letters next to each other).
'black' becomes 'b' (as in pencil classifications).
'very' with its last letter removed is 'ver'.
'modal'+'ver'+'b'='MODAL VERB'
(Other definitions for modal verb that I've seen before include "Word such as can, could, may, etc" , "will possibly" .)