Clue: Key groups including four subordinate to American Navy Seals? (10)
Answer: CARNIVORES
Explanation: Definition: Seals?
An envelope ('including') of A ('American') plus RN (Royal 'Navy' – $$not$$ American) plus IV (Roman numeral ('four') in CORES ('key groups'). 'Subordinate' gives the order of the particles in the down light, and the question mark gives the indication by example.