He was first to show gravity of world situation (5,6)
I believe the answer is:
isaac newton
'he was first to show gravity of world situation' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he was first to' is the wordplay.
I cannot quite understand how this works, but
'he' could be 'newton' and 'newton' is found within the answer.
'was' could be 'is' (being is a kind of being) and 'is' is found within the answer.
'first' could be 'a' (first letter of the alphabet) and 'a' is located in the answer.
The remaining letters 'ac' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for isaac newton that I've seen before include "Seventeenth to eighteenth century English physicist/mathematician" , "Laws of Motion man" , "noted scientist" , "Lawmaker" , "Principia author" .)