Quick - agent’s about to nod off (6)
I believe the answer is:
snappy
'quick' is the definition.
(thesaurus)
'agent's about to nod off' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'about' is an insertion indicator.
'to nod off' becomes 'nap' (I can't justify this - if you can you should believe this answer much more).
'spy' placed around 'nap' is 'SNAPPY'.
Can you help me to learn more?
(Other definitions for snappy that I've seen before include "Ratty" , "Apt to speak irritably: smart and fashionable" , "Quick(-tempered)" , "Instantaneous" , "Irritable - smart" .)