... may invert audiovisual signals - in extremis, seek support (7)
I believe the answer is:
canvass
'seek support' is the definition.
(as in to canvass for votes)
'may invert audiovisual signals in extremis' is the wordplay.
'may' becomes 'can' (synonyms).
'invert audiovisual' becomes 'va' (I can't justify this - if you can you should give a lot more credence to this answer).
'in extremis' says to hollow out the word (remove centre letters) (the extremes of the word are used).
'signals' with its middle removed is 'ss'.
'can'+'va'+'ss'='CANVASS'
Can you help me to learn more?
(Other definitions for canvass that I've seen before include "Seek for votes" , "Appeal for support" , "Solicit votes from (electors)" , "campaign" , "Search for support" .)