Mark surfaced ill-advisedly? Not altogether (7)
I believe the answer is:
cedilla
'mark' is the definition.
(I have seen 'Diacritical mark ' mean 'cedilla' so perhaps 'mark' could also mean 'cedilla')
'surfaced ill-advisedly? not altogether' is the wordplay.
'not altogether' indicates the answer is hidden within the clue.
'CEDILLA' is hidden in the letters of 'surfaced ill advisedly'.
(Other definitions for cedilla that I've seen before include "Low mark in French" , "mark not seen in 18 [FACADE]?" , "Mark under the letter C in French" , "Mark under a consonant in French, perhaps" , "Mark indicating a softened consonant" .)