Ate up most of ham, in reality (7)
I believe the answer is:
defacto
'reality' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'ate up most of ham' is the wordplay.
'ate' becomes 'fed' ('feed' can be a synonym of 'eat').
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'most of' means to remove the last letter.
'ham' becomes 'actor' (I've seen this before).
'fed' reversed gives 'def'.
'actor' with its final letter taken off is 'acto'.
'def'+'acto'='DEFACTO'
'in' acts as a link.
Can you help me to learn more?
(Another definition for defacto that I've seen is " Actually".)