Fix mere dent, I fancy (9)
I believe the answer is:
determine
'fix' is the definition.
(I know that choose is a is a more specific form of the action of fix)
'mere dent i fancy' is the wordplay.
'fancy' indicates anagramming the letters.
'mere'+'dent'+'i'='meredenti'
'meredenti' anagrammed gives 'DETERMINE'.
(Other definitions for determine that I've seen before include "I'm entered, so find our precisely" , "Establish" , "Decide - to settle" , "resolve" , "Choose" .)