Deer overturned cask leaking contents (4)
I believe the answer is:
elks
'deer' is the definition.
(I've seen this before)
'overturned cask leaking contents' is the wordplay.
'overturned' shows that the letters should be reversed in order.
'contents' indicates a hidden word (I've seen 'contents of' mean this).
'skle' is hidden in the letters of 'cask leaking'.
'skle' written backwards gives 'ELKS'.
(Other definitions for elks that I've seen before include "Moose(s)" , "Large deer of Europe and N America" , "Moose" , "Large deer of Northern Europe" , "Large deer with flattened antlers" .)