Go back briefly, then off out to lunch, having these? (4,2,3,6)
I believe the answer is:
bats in the belfry
'out to lunch having these?' is the definition.
('out to lunch' and the answer can both mean mad)
'go back briefly then off' is the wordplay.
'go' becomes 'stab' (both can mean an attempt**).
'back' says the letters should be written backwards.
'off' is an anagram indicator.
'stab' written backwards gives 'bats'.
'briefly'+'then'='brieflythen'
'brieflythen' anagrammed gives 'inthebelfry'.
'bats'+'inthebelfry'='BATS-IN-THE-BELFRY'
(Another definition for bats in the belfry that I've seen is " Slightly mad".)