Pop back, once having got to the top (4)
I believe the answer is:
apex
'to the top' is the definition.
(apex is a top or peak)
'pop back once' is the wordplay.
'pop' becomes 'pa' (both can mean father).
'back' shows that the letters should be reversed in order.
'once' becomes 'ex' (prefix meaning former or onetime).
'pa' written backwards gives 'ap'.
'ap'+'ex'='APEX'
'having got' acts as a link.
(Other definitions for apex that I've seen before include "white collar union" , "Culmination" , "Top point" , "Cheap fare" , "Topmost point" .)