Billions invested in English medical implant (5)
I believe the answer is:
embed
'implant' is the definition.
(synonyms)
'billions invested in english medical' is the wordplay.
'billions' becomes 'b'.
'invested in' means one lot of letters goes inside another.
'english' becomes 'e' (abbreviation).
'medical' becomes 'med' (common abbreviation).
'e'+'med'='emed'
'b' placed inside 'emed' is 'EMBED'.
(Other definitions for embed that I've seen before include "Fix firmly (in ground)" , "Become firmly fixed" , "Fix firmly and deeply" , "Plant deeply" , "Fix fast in something solid" .)