Be unhappy about eating second course (5)
I believe the answer is:
epsom
'course' is the definition.
(a race course)
'be unhappy about eating second' is the wordplay.
'be unhappy' becomes 'mope' (I can't justify this - if you can you should give a lot more credence to this answer).
'about' shows that the letters should be reversed in order.
'eating' means one lot of letters goes inside another.
'second' becomes 's' (common abbreviation).
'mope' in reverse letter order is 'epom'.
'epom' going around 's' is 'EPSOM'.
Can you help me to learn more?
(Other definitions for epsom that I've seen before include "English town, of course?" , "Mopes around in English horse-racing venue" , "- Salts are a purgative" , "Mopes around English Derby venue" , "Classic racecourse" .)