Sentence:  When Johnny Mercer and Harold Arlen began their <WSD>collaboration</WSD> in 1940 , Mercer , like Arlen , had several substantial film songs to his credit , among them `` Hooray for Hollywood '' , `` Ride , Tenderfoot , Ride '' , `` Have You Got Any Castles , Baby ? '' 
Meaning of the word marked with <WSD> tags:  act of working jointly