One fairly quietly spoken may be spoilt (8)
I believe the answer is:
impaired
'be spoilt' is the definition.
(I've seen this before)
'one fairly quietly spoken' is the wordplay.
'one' becomes 'i' (Roman numeral).
'fairly quietly' becomes 'mp' (musical abbreviation for mezzo-piano).
'spoken' becomes 'aired'.
'i'+'mp'+'aired'='IMPAIRED'
'may' is the link.
(Other definitions for impaired that I've seen before include "Had negative impact" , "hurt" , "Damaged, marred" , "Defective" , "Faulty" .)