I'm not sure if you're looking for a way of learning them or if
you're looking for a dictionary where you can look up those you
don't know. If it's the first, you should check out
FluentFlix.com, which has as "Essential Chengyu" section that
teaches and explains Chengyu in an entertaining and informative
way (something I haven't really found elsewhere). If it's
dictionaries you're after, Zdic has a pretty good one (Chinese-
Chinese). Many good explanations with examples can also be found
on Baidu (just search for the Chengyu, here is an example). This
dictionary from Taiwan's MoE is quite good, too. If you want
explanations in English, your choices are more limited, but MDBG
works most of the time, along with the one on Chinese-Tools
already recommended. Finally, if you can't find a Chengyu, I
suggest using Google. Just as in English, if you search for
rather uncommon search terms, the first few hits will be
dictionaries explaining the search term (in Chinese, most
likely). If you still don't find anything, tr