Make useless vain flapping over Liberal, I see (10)
I believe the answer is:
invalidate
'make useless' is the definition.
(to make useless or invalid)
'vain flapping over liberal i see' is the wordplay.
'flapping' indicates an anagram (flap can mean to move or fly around).
'over' is a charade indicator (letters next to each other).
'liberal' becomes 'l' (abbreviation).
'see' becomes 'date' (synonyms).
'vain' anagrammed gives 'inva'.
'inva'+'l'+'i'+'date'='INVALIDATE'
(Other definitions for invalidate that I've seen before include "Cancel" , "End effectiveness" , "Nullify" , "Disprove" , "Make nothing of" .)