Structure of speech announcing duty on beer and fags? (6)
'structure of speech' is the definition.
'announcing duty on beer and fags?' is the wordplay.
'announcing' indicates a 'sounds like' (homophone) clue.
'duty on beer and fags?' becomes 'sintax' (tax on items regarded as sinful or bad).
'sintax' sounds like 'SYNTAX'.
(Other definitions for syntax that I've seen before include "The rules governing the arranging of words", "Verbal order", "The rules governing the arrangement of words in sentences etc", "branch of linguistics", "Grammatical rules".)