### Clue: With 26 across, often found quietly replacing leading firms (5)
### Answer: POUND 
 ### Explanation: Definition: With 26 across, often
 "p"(abbrev. for "piano", musical instruction to play quietly) **replacing ****1st letter of**(leading) "firms", in "found".
Answer: A pound shop(answer to 26 across) is one selling inexpensive merchandise, costing £1 each.
I'm not sure whether I've got the intended definition.