Extra short on top of beer brings confidence (6)
I believe the answer is:
morale
'confidence' is the definition.
(morale can mean spirit or confidence)
'extra short on top of beer' is the wordplay.
'extra short on top of' becomes 'mor' (I can't justify this - if you can you should believe this answer much more).
'beer' becomes 'ale' (type of beer).
'mor'+'ale'='MORALE'
'brings' acts as a link.
(Other definitions for morale that I've seen before include "General level of confidence or optimism" , "State of a group's confidence and spirit" , "Esprit, degree of confidence" , "external confidence" , "Confidence, sense of wellbeing" .)