Clue: Like woeful number produced by adding constant to one third, oddly (9)
Answer: THRENODIC
Explanation: Definition: Like woeful number 
 C(abbrev. for an unspecified number that remains constant/invariable, in maths, say) **placed after**(adding … to) **anagram of**(…, oddly) [ ONE THIRD ].
Defn: …/a threnody/a lament.