Best not to open this farm — no cows unfortunately! (3,2,5)
I believe the answer is:
can of worms
'best not to open this' is the definition.
(a can of worms is a controversial issue)
'farm no cows unfortunately' is the wordplay.
'unfortunately' indicates an anagram.
'farm'+'no'+'cows'='farmnocows'
'farmnocows' anagrammed gives 'CAN OF WORMS'.
(Other definitions for can of worms that I've seen before include "Potential trouble" , "Complex problem" , "amateur angler's bait?" , "Something likely to prove awkward" , "which shouldn't be opened" .)