Position of Pearse, initially, at top of O'Connell Street (4)
I believe the answer is:
post
'position' is the definition.
(post is a kind of position)
'pearse initially at top of o'connell street' is the wordplay.
'initially' says to take the initial letters.
'at' means one lot of letters go next to another.
'top of' suggests taking the first letters.
'street' becomes 'st'.
The initial letter of 'pearse' is 'p'.
The first letter of 'oconnell' is 'o'.
'p'+'o'+'st'='POST'
'of' acts as a link.
(Other definitions for post that I've seen before include "Prefix meaning after" , "Job; upright timber" , "Send via the mail" , "Member < to comment on website" , "Put a letter in the box - stop it" .)