Everyone's turned up with a full picnic! (3,5)
I believe the answer is:
all there
'a full picnic' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'everyone's turned up' is the wordplay.
'everyone' becomes 'all' (I've seen this before).
'turned up' becomes 'there' (I can't justify this - if you can you should give a lot more credence to this answer).
'all'+'there'='ALL THERE'
'with' acts as a link.
Can you help me to learn more?
(Other definitions for all there that I've seen before include "on one's trolley, presumably?" , "Nothing missing" , "Everyone attending" , "Having one's wits about one (informal)" , "quite astute" .)