Clue: Posh brand you order when it's your 19? (3,1,5)
Answer: BUY A ROUND
Explanation: Definition:  

***(u brand you)** [anag:*order*] where U is "posh"
 

 
I think the "order" in the clue should be part of the definition, but then it would be doing double duty as it is necessary to indicate the anagram.