Centre of diameter in fog (5)
I believe the answer is:
midst
'centre' is the definition.
(I've seen this before)
'diameter in fog' is the wordplay.
'diameter' becomes 'd' (mathematics).
'in' means one lot of letters goes inside another.
'fog' becomes 'mist' (mist is a kind of fog).
'd' inserted inside 'mist' is 'MIDST'.
'of' is the link.
(Other definitions for midst that I've seen before include "Blue dye; rainbow hue" , "Where one is surrounded by other things" , "Centre" , "Among, amid" , "In the middle of (archaic or literary)" .)