Terrible monster or a scientist (10)
I believe the answer is:
astronomer
'scientist' is the definition.
(I know that astronomer is a type of physicist)
'terrible monster or a' is the wordplay.
'terrible' indicates an anagram.
'monster'+'or'+'a'='monsterora'
'monsterora' is an anagram of 'ASTRONOMER'.
(Other definitions for astronomer that I've seen before include "Observer of space" , "Stargazer" , "scientist" , "Moon starer (anag.)" , "Dorado may be contemplated by me" .)