Not just "a" (3)
I believe the answer is:
the
'not just a' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for the that I've seen before include "Born - - - - - - - - (carol)" , "Over - - moon" , "and 24 Down: Similar things" , "SECOND PART OF 23" , "Base of ever-expanding nursery rhyme" .)