Without acceleration due to gravity, earth is spherical (5)
I believe the answer is:
round
'spherical' is the definition.
('round' can be similar in meaning to 'spherical')
'without acceleration due to gravity earth' is the wordplay.
'without' indicates named letters should be taken away.
'acceleration due to gravity' becomes 'g' (as in G-force).
'earth' becomes 'ground' ('ground' can be a synonym of 'earth').
'ground' with 'g' removed is 'ROUND'.
'is' acts as a link.
(Other definitions for round that I've seen before include "regular series" , "session" , "Having a circular shape" , "... entire" , "Approximate" .)