Just about beside myself (4)
I believe the answer is:
mere
'just' is the definition.
(I've seen this in other clues)
'about beside myself' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'beside' says to put letters next to each other (I've seen this in other clues).
'myself' becomes 'me'.
're' after 'me' is 'MERE'.
(Other definitions for mere that I've seen before include "Lake (poetic); Maori war club" , "Pure; lake" , "Just an English lake" , "Nothing more than is specified - or a poetic lake" , "bathing may be permitted in this?" .)