Clue: Terribly bad manners, drinking gallons: a Sirah's drained with meal (7,3,4)
Answer: BANGERS AND MASH
Explanation: Definition: meal
An envelope ('drinking') of G ('gallons') in BANERSANDM, an anagram ('terribly') of 'bad manners' plus 'a' plus SH ('**S**ira**H**'s drained').