Crowded; meet up in top ground (7)
I believe the answer is:
teeming
'crowded' is the definition.
(thesaurus)
'meet up in top ground' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'top' suggests taking the first letters.
The first letter of 'ground' is 'g'.
'meet' written backwards gives 'teem'.
'teem'+'in'+'g'='TEEMING'
(Other definitions for teeming that I've seen before include "Alive" , "Chock-a-block" , "full of creatures?" , "(Of rain) falling heavily" , "Full of, abounding in" .)