### Clue: In top half of missile, burn oxygen fuel (7)
### Answer: BUTANOL 
 ### Explanation: Definition: fuel
  [TAN(to burn by being in the sun) + O(symbol for oxygen in chemistry)] **contained in**(in) BUL(first half,top half in a down clue, of "bullet",missile). 
Answer:  Aka butyl alcohol.  It may substitute for petrol in motorcars, and can be produced biologically.