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 give a lot more credence to this answer).
'with' is an insertion indicator.
'agent' becomes 'spy' (spy is a kind of agent).
'set up' says the letters should be written backwards (in a down clue, letters go up).
'spy' backwards is 'yps'.
'gy' placed around 'yps' is 'GYPSY'.
'to take the place of a' is the 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" .)