Bishop heeds petition condemned as irrelevant (6,3,5)
I believe the answer is:
beside the point
'irrelevant' is the definition.
(I've seen this before)
'bishop heeds petition condemned' is the wordplay.
'bishop' becomes 'b' (abbreviation used in chess).
'condemned' is an anagram indicator.
'heeds'+'petition'='heedspetition'
'heedspetition' anagrammed gives 'esidethepoint'.
'b'+'esidethepoint'='BESIDE THE POINT'
'as' is the link.
(Other definitions for beside the point that I've seen before include "as the end of sentence?" , "Sentence starts or ends here" , "Irrelevant" , "It's immaterial" .)