Promise part of hospital will get oxygen for American (4)
I believe the answer is:
word
'promise' is the definition.
(as in 'I give you my word')
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for word that I've seen before include "See 19" , "Promise - unit of language" , "Message" , "Promise; news" , "Puzzle with solutions reading the same down as across" .)