A retreat under yours truly, most unpleasant (7)
I believe the answer is:
meanest
'most unpleasant' is the definition.
(I've seen this before)
'a retreat under yours truly' is the wordplay.
'retreat' becomes 'nest' (nest is a kind of retreat).
'under' says to put letters next to each other (in a down clue, some letters go below others).
'yours truly' becomes 'me' (the person who set this crossword).
'a'+'nest'='anest'
'anest' put after 'me' is 'MEANEST'.
(Other definitions for meanest that I've seen before include "Art expressing" , "Most miserable" , "ultimate in shabbiness?" , "of least value" , "Most stingy, like 16" .)