### Clue: Operations Research used by popular old firm – flower of America (7)
### Answer: ORINOCO 
 ### Explanation: Definition: flower of America
 OR(abbrev. for "Operations Research") **plus**(used by) IN(popular;in fashion) + O(abbrev. for "old") + CO(abbrev. for "company";a commercial firm).
Defn: A flow-er;river in South America.