quite a chore; not that easy; fairly difficult
Colloquial phrase expressing that something is surprisingly or unexpectedly hard work or more difficult than it looks.
Cooking for yourself every day is quite a chore, isn't it?
This problem was fairly difficult, but I managed to solve it.
大変 alone means 'very' or 'difficult'; 結構大変 adds a nuance of 'surprisingly' or 'more than expected'.
結構 can mean 'quite' or 'surprisingly'; here it intensifies 大変 to mean 'quite a lot of trouble'.
Standard kanji-kana mixed spelling.
Kana spelling for 結構; common in casual writing.
Combination of 結構 (けっこう, 'quite, surprisingly') and 大変 (たいへん, 'difficult, hard'). The phrase is a natural collocation, not a fixed idiom.