Blue Danube is in this outing – hired Brendan in two cases (6,4)
I believe the answer is:
triple time
'blue danube' is the definition.
The definition suggests a singular noun which matches the answer.
'this outing hired brendan in two cases' is the wordplay.
'this outing' becomes 'trip' (I've seen this before).
'hired' becomes 'let' (I've seen this in other clues).
'brendan' becomes 'I' (where the crossword setter is 'Brendan').
'in two cases' becomes 'me' (I can't explain this - if you can you should give a lot more credence to this answer).
'trip'+'let'+'i'+'me'='TRIPLE TIME'
'is in' is the link.
Can you help me to learn more?
(Other definitions for triple time that I've seen before include "a very high rate of pay in unsocial hours?" , "What waltzers keep" .)