The one or the other unit covered in solvent (6)
I believe the answer is:
either
'the one or the other' is the definition.
(either one of two things)
'unit covered in solvent' is the wordplay.
'unit' becomes 'i' (resembles 1, a unit).
'covered in' means one lot of letters goes inside another.
'solvent' becomes 'ether' (I've seen this before).
'i' placed inside 'ether' is 'EITHER'.
(Other definitions for either that I've seen before include "Word introducing first of two 16s" , "Word introducing first alternative" , "Each of two; moreover" , "Word indicating the following alternatives" , "No matter which (of two)" .)