Famous scientist working to follow amphibian (6)
I believe the answer is:
newton
'famous scientist' is the definition.
(I've seen this before)
'working to follow amphibian' is the wordplay.
'working' becomes 'on' (e.g. a machine is working or on).
'to follow' says to put letters next to each other (some letters go after or follow others).
'amphibian' becomes 'newt' (newt is a kind of amphibian).
'on' after 'newt' is 'NEWTON'.
(Other definitions for newton that I've seen before include "He discovered the law of gravity" , "Formulator of the law of gravitation, d. 1727" , "Famous mathematician" , "Scientist formulating laws of motion" , "He is said to have discovered gravity" .)