Powerless to accept the Guardian is not fit for purpose (8)
I believe the answer is:
unusable
'not fit for purpose' is the definition.
(I've seen this before)
'powerless to accept the guardian' is the wordplay.
'powerless' becomes 'unable' (similar in meaning).
'to accept' means one lot of letters goes inside another.
'the guardian' becomes 'us' (the crossword setter and their Guardian colleagues).
'unable' placed around 'us' is 'UNUSABLE'.
'is' is the link.
(Other definitions for unusable that I've seen before include "Not fit to employ" , "Of no utility" , "Of no practical benefit" , "Not fit for purpose" , "Not fit to be employed" .)