Great day! Old boy needs raincoat going up country (8)
I believe the answer is:
cambodia
'country' is the definition.
(I know that Cambodia is a country)
'great day old boy needs raincoat going up' is the wordplay.
'great day' becomes 'aid' (I can't justify this - if you can you should believe this answer much more).
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'needs' means one lot of letters go next to another.
'raincoat' becomes 'mac' (macintosh is a type of raincoat).
'going up' is a reversal indicator (in a down clue, the letters are written upwards).
'aid'+'ob'+'mac'='aidobmac'
'aidobmac' backwards is 'CAMBODIA'.
Can you help me to learn more?
(Other definitions for cambodia that I've seen before include "Country of SW Asia, capital Phnom Penh" , "Southeast Asian country" , "somewhere in Asia" , "SE Asian country" , "South East Asian country" .)