Odd vigil held by distraught nun (6)
I believe the answer is:
uneven
'odd' is the definition.
(odd rather than even)
'vigil held by distraught nun' is the wordplay.
'vigil' becomes 'eve' (I've seen this in another clue).
'held by' means one lot of letters goes inside another.
'distraught' is an anagram indicator (an unsettled or upset order of the letters).
'nun' with letters rearranged gives 'unn'.
'eve' going into 'unn' is 'UNEVEN'.
(Other definitions for uneven that I've seen before include "Not smooth or flat" , "Not flat or level" , "Rough, jagged" , "That's odd" , "Rough, bumpy" .)