Declare the north of Avon is actually west! (4)
I believe the answer is:
avow
'declare the north of avon' is the definition.
'avow' can be an answer for 'declare' (avowing is a kind of declaring). I am not sure about the 'the north of avon' bit.
'actually west' is the wordplay.
'actually' becomes 'avo' (I can't justify this - if you can you should believe this answer much more).
'west' becomes 'W' (abbreviation).
'avo'+'w'='AVOW'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for avow that I've seen before include "Assert, swear" , "acknowledge" , "Declare - affirm" , "Assert/confess openly" , "Make a promise" .)