Surprise - it's another small vessel! (4)
I believe the answer is:
stun
'surprise' is the definition.
(stunning is a kind of surprising)
'another small vessel' is the wordplay.
'another small' becomes 's' (abbreviation - e.g. clothes size. I am not sure about the 'another' bit.).
'vessel' becomes 'tun' (type of vessel for beer).
's'+'tun'='STUN'
'it's' is the link.
Can you help me to learn more?
(Other definitions for stun that I've seen before include "Stupefy - daze" , "Concuss; amaze" , "Confound" , "Daze, knock senseless" , "Knock unconscious" .)