Mood of agent encountered on return (6)
I believe the answer is:
temper
'mood' is the definition.
(temper is one's mood or state of mind)
'agent encountered on return' is the wordplay.
'agent' becomes 'rep' (short for representative).
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'on return' shows that the letters should be reversed in order.
'rep'+'met'='repmet'
'repmet' back-to-front is 'TEMPER'.
'of' is the link.
(Other definitions for temper that I've seen before include "Sudden outburst of anger" , "Display of anger" , "bait" , "Paddy - moderate" , "Tantrum" .)