Fail to consume junk food (4,7)
I believe the answer is:
come unstuck
'fail' is the definition.
(I've seen this before)
'consume junk food' is the wordplay.
'junk' indicates an anagram.
'food' becomes 'tuck' (I've seen this before).
'consume' is an anagram of 'comeuns'.
'comeuns'+'tuck'='COME UNSTUCK'
'to' acts as a link.
(Other definitions for come unstuck that I've seen before include "detached?" , "Fail" , "Suffer failure" , "experience a setback?" , "Lose one's grip" .)