Gordonstoun's principal pursuing delicate matter (5)
I believe the answer is:
thing
'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 "Individual unit" , "Article" , "Affair - matter" , "Object, artefact" , "Inanimate object" .)