In 大辞泉 (via goo here) at the first line I found the phrase
What is the meaning of 副助? Can not find it..
Dictionaries usually list abbreviations. 副助 stands for 副助詞 "adverbial particle", i.e. [副助] = [adv. part.].
The suffix 詞 is added to all parts of speech and is omitted in the abbreviations. There are also 副詞 "adverb" and 助詞 "particle" which would be abbreviated [副] and [助].