Agent disturbed by swindles, overcome with horror (6)
I believe the answer is:
aghast
'overcome with horror' is the definition.
'agent disturbed by swindles' is the wordplay.
'agent' becomes 'agt' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'disturbed by' means one lot of letters goes inside another (inserted letters disrupt or disturb the word).
'swindles' becomes 'has' (have can mean to con or swindle).
'agt' going around 'has' is 'AGHAST'.
Can you help me to learn more?
(Other definitions for aghast that I've seen before include "in awe" , "Taken aback with horror" , "Shocked, dumbfounded" , "Horror-stricken, filled with consternation" , "Stunned" .)