Idiot — there’s just one H in “spheroid” (8)
I believe the answer is:
bonehead
'idiot' is the definition.
(both can mean a stupid person)
'just one h in spheroid' is the wordplay.
'just one' becomes 'one' (only one).
'in' is an insertion indicator.
'spheroid' becomes 'bead' (a bead of liquid).
'one'+'h'='oneh'
'oneh' inserted inside 'bead' is 'BONEHEAD'.
'there's' acts as a link.
(Other definitions for bonehead that I've seen before include "Stupid person (colloq.)" , "stupid leader" , "Stupid person with ossified cranium" , "Numbskull" , "Stupid person with ossified brain" .)