Presence of posh bishop that is extremely timely (6)
I believe the answer is:
ubiety
'presence' is the definition.
(I know that ubiety is a type of presence)
'posh bishop that is extremely timely' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'bishop' becomes 'B' (abbreviation for bishop in chess).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'extremely' means to remove the middle letters (only the extremes of the word are used).
'timely' with its middle taken out is 'ty'.
'u'+'b'+'ie'+'ty'='UBIETY'
'of' is the link.
(Other definitions for ubiety that I've seen before include "Whereness" , "location" .)