Grammar slip that's not rocket surgery (5,8)
I believe the answer is:
mixed metaphor
'grammar slip that's not rocket surgery' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mixed metaphor that I've seen before include "Ridiculous combination of two figures of speech" , ""A rolling stone is worth two in the bush", for example" , "Incompatible figure of speech" , "Ridiculous image" , "Muddled figure of speech" .)