When agent returns, I'll detail danger (5)
I believe the answer is:
peril
'danger' is the definition.
(synonyms)
'when agent returns' is the wordplay.
'when' becomes 'li' (I am not sure about this - if you are sure you should believe this answer much more).
'agent' becomes 'rep' (short for representative).
'returns' is a reversal indicator.
'li'+'rep'='lirep'
'lirep' in reverse letter order is 'PERIL'.
'i'll detail' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for peril that I've seen before include "Quest" , "Great danger" , "Hazardous situation" , "Danger, hazard" , "Exposure to harm" .)