Nine unclued lights can be separated into three groups; each group consists of three definitions of one of three words into which the remaining unclued light can be separated.
 

Across

 
1    Estate wild spree devastates (8)
6    Bird with tail obscured by country shrub (6)
10    Fellow, one feels sadly, holding role in retreat that can be clearly shown (12)
11    Old sweets, no end of lemon drops (5)
13    Measure day in life (7)
14    Operating illegally in desert in confusion (6)
16    Locate quote in speech (4)
17    Oval base designed for basins (8)
21    Cattle thieves no longer...