Drop fruit on physicist's head (5)
I believe the answer is:
plump
'drop' is the definition.
(I know that plump is a type of drop)
'fruit on physicist's head' is the wordplay.
'fruit' becomes 'plum' (plum is a kind of fruit).
'on' says to put letters next to each other.
'head' suggests taking the first letters.
The first letter of 'physicists' is 'p'.
'plum'+'p'='PLUMP'
(Other definitions for plump that I've seen before include "Opt - round" , "Well rounded - opt suddenly" , "Slightly fat" , "Portly" , "Choose; chubby" .)