Across end of difficult plain (5)
I believe the answer is:
overt
'plain' is the definition.
(both can mean visible or obvious)
'across end of difficult' is the wordplay.
'across' becomes 'over' (synonyms).
'end of' says to take the final letters.
The final letter of 'difficult' is 't'.
'over'+'t'='OVERT'
(Other definitions for overt that I've seen before include "Open and observable" , "Plain and obvious" , "Plainly apparent" , "Clear" , "Open,up front" .)