A fool to assume the solution (6)
I believe the answer is:
donkey
'a fool' is the definition.
(I've seen this before)
'assume the solution' is the wordplay.
'assume' becomes 'don' (I've seen this in another clue).
'the solution' becomes 'key' (I've seen this in another clue).
'don'+'key'='DONKEY'
'to' is the link.
(Other definitions for donkey that I've seen before include "See 21" , "dope" , "Animal for drudge work" , "Domestic animal" , "Beasts of burden" .)