Very unpleasant engineer least seen in British yard (7)
I believe the answer is:
beastly
'very unpleasant' is the definition.
(I've seen this before)
'engineer least seen in british yard' is the wordplay.
'engineer' indicates anagramming the letters.
'seen in' means one lot of letters goes inside another.
'british' becomes 'b' (abbreviation e.g. in BBC).
'yard' becomes 'y' (abbreviation).
'least' anagrammed gives 'eastl'.
'b'+'y'='by'
'eastl' inserted within 'by' is 'BEASTLY'.
(Other definitions for beastly that I've seen before include "Bestial" , "Horrid" , "Brutal - foul" , "Very unpleasant" , "Brutish" .)