In the end, still reckon Posh should be involved (10)
I believe the answer is:
eventually
'in the end' is the definition.
(I've seen this before)
'still reckon posh should be involved' is the wordplay.
I cannot quite understand how this works, but
'still' could be 'even' ('even' can be a synonym of 'still') and 'even' is located in the answer.
'posh' could be 'u' (abbreviation for upper-class - opposite to non-u) and 'u' is found within the answer.
'be' could be 'tally' (tallying is a kind of being) and 'tally' is found in the remaining letters.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for eventually that I've seen before include "In the end" , "At last" , "Finally" , "In the fullness of time" , "In due course" .)