Declare to be a promise (4)
I believe the answer is:
avow
'promise' is the definition.
(I've seen this before)
'declare to be a' is the wordplay.
'declare' becomes 'vow' (I've seen this in another clue).
'to' means one lot of letters go next to another (I've seen this in other clues).
'vow' put after 'a' is 'AVOW'.
(Other definitions for avow that I've seen before include "Assert or confess openly" , "Declare - admit" , "acknowledge" , "Affirm, swear" , "Solemnly declare" .)