Most difficult son with ultimate challenge (8)
I believe the answer is:
severest
'most difficult' is the definition.
(I've seen this before)
'son with ultimate challenge' is the wordplay.
'son' becomes 's'.
'with' means one lot of letters go next to another.
'ultimate challenge' becomes 'everest' (tallest mountain in the world).
's'+'everest'='SEVEREST'
(Other definitions for severest that I've seen before include "Most dangerous" , "Most extreme or difficult" , "Most strict and harsh" , "Most harsh and punitive" .)