Tired perhaps, but turn over to be cheerful (6)

'tired' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.

'but turn over to be cheerful' is the wordplay.
'but' becomes 'bea' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'turn' becomes 't' (abbreviation in Oxford Dictionary of Abbreviations).
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'to be cheerful' becomes 'up' (up can mean cheerful or upbeat).
'bea'+'t'+'up'='BEATUP'

'perhaps' is the link.

Can you help me to learn more?