Junk - resort to a bit of mockery (4)
I believe the answer is:
spam
'junk' is the definition.
(I've seen this before)
'resort to a bit of mockery' is the wordplay.
'resort' becomes 'spa' (spa is a kind of resort).
'to a bit of' says to take the initial letters.
The initial letter of 'mockery' is 'm'.
'spa'+'m'='SPAM'
(Other definitions for spam that I've seen before include "Tinned processed meat" , "Unwanted communication" , "Possible item from menu" , "unwanted correspondence" , "Tinned meat - unwanted email" .)