Corporation keeping hospital billions in hitch (5)
I believe the answer is:
thumb
'hitch' is the definition.
(hitch-hiking)
'corporation keeping hospital billions' is the wordplay.
'corporation' becomes 'tum' (I've seen this before).
'keeping' means one lot of letters goes inside another.
'hospital' becomes 'h'.
'billions' becomes 'b'.
'tum' enclosing 'h' is 'thum'.
'thum'+'b'='THUMB'
'in' acts as a link.
(Other definitions for thumb that I've seen before include "Hitch-hiker's tool" , "could do it for a lift" , "Tom's digit" , "Short thick manual digit" , "Digit that enables grasping" .)