Open letter to Don, possibly (4)
I believe the answer is:
undo
'open' is the definition.
(eg opening a bottle)
'letter to don possibly' is the wordplay.
'letter' becomes 'u' ().
'to' means one lot of letters go next to another.
'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" .)