Back rank is just not good enough (6-4)
I believe the answer is:
second-rate
'just not good enough' is the definition.
'back rank' is the wordplay.
'back' becomes 'second' (synonyms).
'rank' becomes 'rate' (synonyms).
'second'+'rate'='SECOND-RATE'
'is' acts as a link.
(Other definitions for second-rate that I've seen before include "Substandard" , "Of mediocre quality" , "Shoddy" , "Poor" , "Inferior" .)