### Clue: Act spitefully as Gaff replaces unionist in the street (6)
### Answer: AVENGE 
 ### Explanation: Definition: Act spitefully
 **The 1st letter of** "Gaff " **replacing**(replaces … in) "u"(abbrev. for "unionist") **in** "avenue"(a broad street often lined with trees).
Answer: To act in retaliation (spite for spite, one might say).