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 wordplay.
'principal' suggests taking the first letters.
'pursuing' means one lot of letters go next to another.
'delicate' becomes 'thin' ('thin' can be similar in meaning to 'delicate').
The first 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".)