Happy with agent set up to take the place of a traveller (5)
I believe the answer is:
gypsy
'traveller' is the definition.
(gypsy is a kind of traveller)
'happy with agent set up' is the wordplay.
'happy' becomes 'gy' (I can't explain this - if you can you should believe this answer much more).
'with' means one lot of letters goes inside another.
'agent' becomes 'spy' (spy is a kind of agent).
'set up' says the letters should be written in reverse (in a down clue, letters go up).
'spy' in reverse letter order is 'yps'.
'gy' enclosing 'yps' is 'GYPSY'.
'to take the place of a' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for gypsy that I've seen before include "Romany" , "old Broadway play" , "Traveller" , "Member of a normadic people" .)