Smart agent grabbing short sleep (6)
I believe the answer is:
snappy
'smart' is the definition.
(snappy can mean smartly dressed)
'agent grabbing short sleep' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'grabbing' means one lot of letters goes inside another.
'short sleep' becomes 'nap' (a nap is a short sleep; 'short' asks for a short form).
'spy' placed around 'nap' is 'SNAPPY'.
(Other definitions for snappy that I've seen before include "Informally brisk" , "Quick(-tempered)" , "(Make it) quick" , "Instantaneous" , "as a croc?" .)