Clue: They emit foul smell – very large gauge reverse of ideal (9)
Answer: OSMETERIA
Explanation: Definition: They emit foul smell
**OS** (outsize, so "very large") + **METER** ("guage") + **<=A1** *[reverse of]* "ideal" 
*Osmoteria are organs in caterpillars that emit foul smells (and/or create a display) to deter predators.