Trendy length uniform adopted by guard to show authority (9)
I believe the answer is:
influence
'authority' is the definition.
(thesaurus)
'trendy length uniform adopted by guard' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'length' becomes 'l' (abbreviation in geometry).
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'adopted by' indicates putting letters inside (I've seen this in other clues).
'guard' becomes 'fence' (guard can mean a protective fence).
'l'+'u'='lu'
'lu' going within 'fence' is 'fluence'.
'in'+'fluence'='INFLUENCE'
'to show' is the link.
(Other definitions for influence that I've seen before include "Agency" , "Clue in fen (anag.)" , "The prestige and power to affect others" , "Weight" , "required to pull strings" .)