Clue: Top Gear produced by serious men from the east (8)
Answer: SOMBRERO
Explanation: Definition: Top Gear
 SOMBRE(serious/solemn) + **reversal of**(… from the east, in an across clue) OR(abbrev. for "other ranks"/men in the military who are non-commissioned).
Defn: An article of clothing/gear for one's head/top.