Fight when teetotaller gets in birthday beer! (2,6)

'fight' is the definition.
(to do battle is to fight)

'teetotaller gets in birthday beer' is the wordplay.
'teetotaller' becomes 'tt' (abbreviation).
'gets in' means one lot of letters goes inside another.
'birthday' becomes 'DOB' (abbreviation for date of birth).
'beer' becomes 'ale' (type of beer).
'dob'+'ale'='dobale'
'tt' put within 'dobale' is 'DO BATTLE'.

'when' acts as a link.