Very eager converts undertake Mass (4,2,7)
I believe the answer is:
keen as mustard
'very eager' is the definition.
(I've seen this before)
'converts undertake mass' is the wordplay.
'converts' indicates an anagram.
'undertake'+'mass'='undertakemass'
'undertakemass' with letters rearranged gives 'KEEN AS MUSTARD'.
(Other definitions for keen as mustard that I've seen before include "Extremely eager" , "Intensely enthusiastic" , "Most zealous" , "Very interested to get involved" .)