Main course served to Americans on board, it's said (6)
I believe the answer is:
entree
'main course served to americans' is the definition.
(originally US term for the main course)
'on board it's said' is the wordplay.
I cannot really see how this works, but
'on' could be 're' ('on' can be mean 'with reference to') and 're' is found within the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
The remaining letters 'ene' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for entree that I've seen before include "A dish allowed admittance" , "Main course (or the one before)" , "Principal dish of a meal" , "Starter" , "Dinner course" .)