Gordonstoun's principal pursuing delicate matter (5)
I believe the answer is:
Here is my best explanation:
'matter' is the definition.
('thing' can be a synonym of 'matter')
'gordonstoun's principal pursuing delicate' is the subsidiary indication.
'principal' says to take the initial letters.
'pursuing' says to put letters next to each other.
'delicate' becomes 'thin' ('thin' can be similar in meaning to 'delicate').
The initial letter of 'gordonstoun' is 'g'.
'g' after 'thin' is 'thing'.
(Other definitions for thing that I've seen before include "Unspecified object", "See 1", "Affair - matter", "Individual object", "Night (anag)", "Individual unit", "Object, item", "An unnamed object or item", "Unnamed object" and "Object; gadget".)