Fix quickly before half open (6)
I believe the answer is:
fasten
'fix' is the definition.
(I know that fix can be written as fasten)
'quickly before half open' is the wordplay.
'quickly' becomes 'fast' (synonyms).
'before' is a charade indicator (letters next to each other).
'half' means to take half the letters.
'open' cut in half is 'en'.
'fast'+'en'='FASTEN'
(Other definitions for fasten that I've seen before include "Bolt" , "Attach" , "Fix securely" , "Affix" , "Close, make secure" .)