Scientist has dropped on amphibian (4)
I believe the answer is:
newt
'amphibian' is the definition.
(newt is a kind of amphibian)
'scientist has dropped on' is the wordplay.
'scientist' becomes 'Newton' (Sir Isaac Newton).
'has dropped' is a deletion indicator.
'newton' with 'on' taken away is 'NEWT'.
(Other definitions for newt that I've seen before include "Semiaquatic amphibian" , "Went (anag.)" , "Amphibious creature" , "Medical worker" , "Went to find amphibian" .)