To summarise, it's about top (5)
I believe the answer is:
recap
'to summarise' is the definition.
(recapping is a kind of summarizing)
'about top' is the wordplay.
'about' becomes 're' (common abbreviation for 'with reference to').
'top' becomes 'cap' (cap is a kind of top).
're'+'cap'='RECAP'
'it's' acts as a link.
(Other definitions for recap that I've seen before include "In brief, go over the main points again" , "look back on past things" , "Summarise briefly (abbr)" , "play again" , "Sum up (abbrev.)" .)