Augment noise of horn by adding occasional flux (4,2)
I believe the answer is:
beef up
'augment' is the definition.
(thesaurus)
'noise of horn by adding occasional flux' is the wordplay.
'noise of horn' becomes 'beep' (a car horn goes beep beep).
'by adding' is an insertion indicator.
'occasional' indicates alternate letters (letters taken at regular intervals or on occasion).
The alternating letters of 'flux' are 'fu'.
'beep' enclosing 'fu' is 'BEEF UP'.
(Other definitions for beef up that I've seen before include "Give (something) more strength" , "message poor meat-eaters won't want to hear?" , "Gain muscle" , "Give added strength" , "buttress" .)