Behind so petite? Bum irrelevant (6,3,5)
I believe the answer is:
beside the point
'irrelevant' is the definition.
(I've seen this before)
'behind so petite? bum' is the wordplay.
'bum' is an anagram indicator (bum can mean faulty or wrong).
'behind'+'so'+'petite'='behindsopetite'
'behindsopetite' with letters rearranged gives 'BESIDE THE POINT'.
(Other definitions for beside the point that I've seen before include "Irrelevant" , "Sentence starts or ends here" , "as the end of sentence?" , "It's immaterial" .)