### Clue: Old fellows against backing Penny, a female prodigy (10)
### Answer: PHENOMENON 
 ### Explanation: Definition: prodigy
 [ O(abbrev. for "old") + MEN(fellows) + ON(against, as in "to lean on the back of the chair") ] **placed after**(backing) [ P(abbrev. for "penny", the currency unit) + HEN(the female of the species) ].