One RBS manager's horribly awkward (12)
I believe the answer is:
embarrassing
'awkward' is the definition.
(I know that awkward can be written as embarrassing)
'one rbs manager's horribly' is the wordplay.
'one' becomes 'i' (Roman numeral).
'horribly' indicates an anagram (a horribly wrong version of the letters).
'i'+'rbs'+'managers'='irbsmanagers'
'irbsmanagers' is an anagram of 'EMBARRASSING'.
(Other definitions for embarrassing that I've seen before include "Causing one to feel awkward" , "Humiliating" , "Blush-making" , "Causing to feel shame" , "Socially very awkward" .)