French article supporting prediction these crew's quarters may be reduced by 40% (10)
I believe the answer is:
forecastle
'these crew's quarters may be reduced by 40%' is the definition.
The definition suggests an adverb but the answer is not.
'french article supporting prediction' is the wordplay.
'french article' becomes 'le' (definite article in French).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'prediction' becomes 'forecast' (forecast is a kind of prediction).
'le' put after 'forecast' is 'FORECASTLE'.
Can you help me to learn more?
(Other definitions for forecastle that I've seen before include "located at the front" , "section of the ship" , "Fo'c'sle in full" , "Part of vessel" , "where the crew is housed" .)