It's a very old wicket, so declare (4)
I believe the answer is:
avow
'declare' is the definition.
(avowing is a kind of declaring)
'it's a very old wicket' is the wordplay.
'it's a very' becomes 'av' (I've seen this in other clues).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'wicket' becomes 'w'.
'av'+'o'+'w'='AVOW'
'so' acts as a link.
(Other definitions for avow that I've seen before include "acknowledge" , "Solemnly promise" , "Assert or confess openly" , "Asseverate" , "Declare a promise of fidelity at the altar" .)