Boot bottom (4)
I believe the answer is:
sole
'boot bottom' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sole that I've seen before include "Unique" , "Underside of the foot" , "Exclusive class of fish" , "A lone flat fish?" , "O __ Mio (Neapolitan song)" .)