Attach if fax is misdirected (5)
I believe the answer is:
affix
'attach' is the definition.
(affixing is a kind of attaching)
'if fax is misdirected' is the wordplay.
'is misdirected' is an anagram indicator (I've seen this in other clues).
'if'+'fax'='iffax'
'iffax' anagrammed gives 'AFFIX'.
(Other definitions for affix that I've seen before include "Stick on" , "Glue" , "Attach" , "Append" , "Join" .)