Buzz Aldrin not half getting acceleration due to gravity (4)
I believe the answer is:
ring
'buzz' is the definition.
(I've seen this before)
'aldrin not half getting acceleration due to gravity' is the wordplay.
'not half' suggests halving the letters.
'getting' says to put letters next to each other.
'acceleration due to gravity' becomes 'g' (as in G-force).
'aldrin' with half the letters taken is 'rin'.
'rin'+'g'='RING'
(Other definitions for ring that I've seen before include "Buzz" , "Piece of jewellery" , "Phone; circular band" , "Telephone - as bells do" , "Wedding band" .)