Open letter to Don, possibly (4)
I believe the answer is:
undo
'open' is the definition.
(I've seen this before)
'letter to don possibly' is the wordplay.
'letter' becomes 'u' ().
'to' says to put letters next to each other.
'possibly' indicates anagramming the letters.
'don' is an anagram of 'ndo'.
'u'+'ndo'='UNDO'
(Other definitions for undo that I've seen before include "Unfasten - destroy" , "Unfasten - ruin" , "Relax" , "Untie or ruin" , "Bring to nought" .)