Clue: Regret having ordered variable, boring Rebus novel about hard-drinking copper (6,7)
Answer: BUYER’S REMORSE
Explanation: Definition: Regret having ordered

**Y** (variable, in mathematics) *boring* ***(rebus)** [anag:*novel*] + **RE** ("about") + **MORSE** ("hard-drinking copper").
 

 
Inspector Morse, in the Colin Dexter books, had a penchant for real ale.