Second chicken drinks beer on top of barn for encouragement … (6-7)

'encouragement' is the definition.
(morale booster is a kind of encouragement)

'second chicken drinks beer on top of barn' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'chicken' becomes 'rooster' (rooster is a kind of chicken).
'drinks' indicates putting letters inside.
'beer on top of barn' becomes 'aleb' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'rooster' enclosing 'aleb' is 'ralebooster'.
'mo'+'ralebooster'='MORALE-BOOSTER'

'for' acts as a link.