Smart, agent snatching short sleep (6)
I believe the answer is:
snappy
'smart' is the definition.
(thesaurus)
'agent snatching short sleep' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'snatching' is an insertion indicator.
'short sleep' becomes 'nap' ('nap' can be a synonym of 'short sleep'**).
'spy' going around 'nap' is 'SNAPPY'.
(Other definitions for snappy that I've seen before include "without hesitation" , "as a croc?" , "Ratty" , "Elegant" , "Irritably abrupt" .)