Sickeningly, bits of alum devalued natural gold layer (2,7)
I believe the answer is:
ad nauseam
'sickeningly' is the definition.
(I've seen this in another clue)
'bits of alum devalued natural gold layer' is the wordplay.
'bits of' suggests taking the first letters.
'gold' becomes 'Au' (Au is the chemical symbol for gold).
'layer' becomes 'seam' (seam is a kind of layer).
The initial letters of 'alum devalued natural' is 'adn'.
'adn'+'au'+'seam'='AD NAUSEAM'
(Other definitions for ad nauseam that I've seen before include "To a very tiresome degree" , "To a disgusting degree" , "Continually" , "Excessively, to a sickening extent" , "Past the stage of enjoying" .)