Clue: Joints engineers knocked back in Sandbanks (7)
Answer: REEFERS
Explanation: Definition: Joints
 **Reversal of**(… knocked back) RE(abbrev. for the Royal Engineers, a unit in the British military) **contained in**(in) REEFS(sandbanks/ridges of sand just above or below the sea surface).
Defn: …/cannabis cigarettes.