It can be bitter for the French, one day (5)
I believe the answer is:
lemon
'it can be bitter' is the definition.
The definition suggests a singular noun which matches the answer.
'the french one day' is the wordplay.
'the french' becomes 'le' ('the' in French).
'one day' becomes 'mon' (abbreviation for Monday).
'le'+'mon'='LEMON'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for lemon that I've seen before include "Feeble thing (colloq.); fruit tree" , "Fruit; sort of grass, sole" , "Fruit - useless thing" , "A fruit; a worthless item" , "Leaving body" .)