More crackers for medium snake (6)
I believe the answer is:
madder
'more crackers' is the definition.
(I've seen this in another clue)
'medium snake' is the wordplay.
'medium' becomes 'm' (as in UK clothing sizes).
'snake' becomes 'adder' (type of snake).
'm'+'adder'='MADDER'
'for' is the link.
(Other definitions for madder that I've seen before include "more bananas" , "Pigment" , "Red dye; more barmy" , "More irate" , "Rot" .)