Possibly uppermost in arrogance (11)
I believe the answer is:
presumption
'arrogance' is the definition.
(I've seen this before)
'possibly uppermost in' is the wordplay.
'possibly' indicates an anagram.
'uppermost'+'in'='uppermostin'
'uppermostin' with letters rearranged gives 'PRESUMPTION'.
(Other definitions for presumption that I've seen before include "Arrogance" , "Supposition - impertinence" .)