Confusion of ‘bail’ and ‘mail’ involves special form of pronunciation (9)

'form of pronunciation' is the definition.
(term in phonetics)

'confusion of bail and mail involves special' is the wordplay.
'confusion of' indicates anagramming the letters (I've seen this in other clues).
'and' is a charade indicator (letters next to each other) (I've seen this in other clues).
'involves' indicates putting letters inside.
'special' becomes 's' (common abbreviation as in Special Air Service).
'bail' after 'mail' is 'mailbail'.
'mailbail' is an anagram of 'labialim'.
'labialim' going around 's' is 'LABIALISM'.