Plug new gold layer to a sickening extent (2,7)
I believe the answer is:
ad nauseam
'to a sickening extent' is the definition.
(synonyms)
'plug new gold layer' is the wordplay.
'plug' becomes 'ad' (as in plugging a product).
'new' becomes 'n' (abbreviation as in New Testament NT).
'gold' becomes 'Au' (Au is the chemical symbol for gold).
'layer' becomes 'seam' (seam is a kind of layer).
'ad'+'n'+'au'+'seam'='AD NAUSEAM'
(Other definitions for ad nauseam that I've seen before include "Interminably" , "To a very tiresome degree" , "Done so often that it becomes tiresome" , "Repeating" , "seasickness pills like this would be counterproductive?" .)