Dissolute master joining ship that's changed sides (10)
I believe the answer is:
profligate
'dissolute' is the definition.
(I know that dissolute can be written as profligate)
'master joining ship that's changed sides' is the wordplay.
I cannot quite see how this works, but
'master' could be 'pro' (I've seen this in another clue) and 'pro' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for profligate that I've seen before include "Wastrel" , "Dissolute" , "Recklessly extravagant" , "Recklessly wasteful" , "Debauched" .)