Certain Arabian chaps occupying the old institute (6)
I believe the answer is:
yemeni
'certain arabian' is the definition.
'Yemeni' can be an answer for 'arabian' (Yemeni is an example). I'm unsure of the 'certain' bit.
'chaps occupying the old institute' is the wordplay.
'chaps' becomes 'men' (chap can mean a man).
'occupying' means one lot of letters goes inside another (inserted letters occupy or enter the word).
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'institute' becomes 'i' (common abbreviation as in MIT).
'ye'+'i'='yei'
'men' inserted into 'yei' is 'YEMENI'.
Can you help me to learn more?
(Other definitions for yemeni that I've seen before include "Person from Sana'a, say" , "Native of, eg, Aden" , "language" , "Inhabitant of the country at the southern end of the Arabia" , "Eg, native of Sana'a" .)