He's on about being present (4)
I believe the answer is:
here
'being present' is the definition.
('here' can be similar in meaning to 'present')
'he's on about' is the wordplay.
'on' means one lot of letters go next to another.
'about' becomes 're' (regarding**).
'he'+'re'='HERE'
(Other definitions for here that I've seen before include "At this stage" , "Present - at this point" , "The present location" , "To hand" , "final clue?" .)