Solution not entirely fully explained (3)
I believe the answer is:
lye
'solution' is the definition.
(I know that lye is a solution)
'not entirely fully explained' is the wordplay.
'not entirely' indicates the answer is hidden within the clue.
'LYE' is hidden in the letters of 'fully explained'.
(Other definitions for lye that I've seen before include "Strong alkaline solution" , "Strongly alkaline solution for washing or cleansing" , "solution that comes out in the wash?" , "Solution obtained by leaching" , "Alkaline liquid" .)