Attention that is halved in this sphere (5)
I believe the answer is:
earth
'this sphere' is the definition.
(I have seen 'Terrestrial sphere ' mean 'earth' so perhaps 'sphere' could also mean 'earth')
'attention that is halved' is the wordplay.
'attention' becomes 'ear' (having someone's ear is having their attention).
'is halved' means to take half the letters.
'that' with half the letters taken is 'th'.
'ear'+'th'='EARTH'
'in' acts as a link.
(Other definitions for earth that I've seen before include "Heart (anag)" , "Land" , "Soil; electrical ground" , "Third rock from the Sun" , "Soil; ground terminal" .)