Cross over, Newton; trouble may be dead as they say (8)
I believe the answer is:
doornail
'be dead as they say' is the definition.
The definition suggests an adverb but the answer is not.
'cross over newton trouble' is the wordplay.
'cross' becomes 'door' (a door is a kind of cross).
'over' means one lot of letters go next to another.
'newton' becomes 'n' (synonyms).
'trouble' becomes 'ail' (synonyms).
'door'+'n'+'ail'='DOORNAIL'
'may' acts as a link.
Can you help me to learn more?
(Other definitions for doornail that I've seen before include "Dead as a ........ is dead without a doubt" , "popularly believed dead" , "Stud" , "proverbially dead" , "Dead as a ...-very dead indeed" .)