A spirit's leading authority put to the proof (5)
I believe the answer is:
assay
'put to the proof' is the definition.
(I've seen this before)
'a spirit's leading authority' is the wordplay.
'leading' indicates taking the first letters.
'authority' becomes 'say' (I've seen this in other clues).
The initial letter of 'spirits' is 's'.
'a'+'s'+'say'='ASSAY'
(Other definitions for assay that I've seen before include "'Analyse, precious metal, say (5)'" , "Test gold" , "Test of substance" , "Check (a metal's) quality" , "Examine (ore or alloy?)" .)