Apt cue could be a drinking vessel! (6)
I believe the answer is:
teacup
'a drinking vessel' is the definition.
(I've seen this before)
'apt cue could be' is the wordplay.
'could be' is an anagram indicator.
'apt'+'cue'='aptcue'
'aptcue' anagrammed gives 'TEACUP'.
(Other definitions for teacup that I've seen before include "Saucer under this?" , "Drinking container that may contain a small storm" , "A storm here is minor" , "Earl Grey holder?" , "proverbially stormy location" .)