Meet going back into top grasslands abounding (7)
I believe the answer is:
teeming
'abounding' is the definition.
(I've seen this before)
'meet going back into top grasslands' is the wordplay.
'going back' shows that the letters should be reversed in order.
'into' becomes 'in'.
'top' indicates taking the first letters (in a down clue, the first letter is top).
The first letter of 'grasslands' is 'g'.
'meet' backwards is 'teem'.
'teem'+'in'+'g'='TEEMING'
(Other definitions for teeming that I've seen before include "Swarming in to meeting" , "Alive" , "(Of rain) falling heavily" , "Full of people" , "Pouring - abounding" .)