Exact straight line passed over centre of gravity (4)
I believe the answer is:
levy
'exact straight line' is the definition.
'levy' can be an answer for 'exact' (I've seen this in another clue). I'm not certain of the 'straight line' bit.
'passed over centre of gravity' is the wordplay.
'passed' becomes 'ley' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'over' means one lot of letters goes inside another.
'centre of' indicates the central letters.
The central letter of 'gravity' is 'v'.
'ley' enclosing 'v' is 'LEVY'.
Can you help me to learn more?
(Other definitions for levy that I've seen before include "Raise - army (or money for it)" , "Raise or collect (a tax)" , "Impost" , "tax money" , "Tax, fee, fine, etc" .)