Like woeful number produced by adding constant to one third, oddly (9)
I believe the answer is:
threnodic
'like woeful number produced by adding' is the definition.
I can't judge whether this definition defines the answer.
'constant to one third oddly' is the wordplay.
'constant' becomes 'c' (common symbol in maths for an unknown constant).
'to' means one lot of letters go next to another (I've seen this in other clues).
'oddly' indicates an anagram (letters in an odd order).
'one'+'third'='onethird'
'onethird' with letters rearranged gives 'threnodi'.
'c' put after 'threnodi' is 'THRENODIC'.
Can you help me to learn more?
(Another definition for threnodic that I've seen is " Like a dirge".)