Make invalid somehow fly in? Extremely unusual (7)
I believe the answer is:
nullify
'make invalid' is the definition.
(I've seen this before)
'somehow fly in? extremely unusual' is the wordplay.
'somehow' indicates an anagram.
'extremely' suggests removing the centre.
'unusual' with its middle taken out is 'ul'.
'fly'+'in'+'ul'='flyinul'
'flyinul' anagrammed gives 'NULLIFY'.
(Other definitions for nullify that I've seen before include "In fully (anag.)" , "Make void" , "Bring to naught" , "Render void" , "Render ineffective" .)