### Clue: Second US president back in A&E — cause of dysentery? (7)
### Answer: AMOEBAE 
 ### Explanation: Definition: cause of dysentery
 [ MO(short for a moment in time/a second) + **reversal of**(… back) ABE(diminutive of the name Abraham, first name of former US president, Lincoln) ] **contained in**(in) [A & E].
Defn: … which are tiny unicellular organisms, a species of which are intestinal parasites.