Equally proficiently, too (2,4)
I believe the answer is:
as well
'too' is the definition.
(synonyms)
'equally proficiently' is the wordplay.
'equally' becomes 'as' (synonyms).
'proficiently' becomes 'we'll' (I've seen this in another clue).
'as'+'well'='AS WELL'
(Other definitions for as well that I've seen before include "Also, in addition" , "on top" , "Also, additionally" , "Appropriate" , "A swell sort of addition" .)