A number disappeared in Turkey on vacation (6)
I believe the answer is:
twenty
'a number' is the definition.
(twenty is a kind of number)
'disappeared in turkey on vacation' is the wordplay.
'disappeared' becomes 'went' (I've seen this in another clue).
'in' is an insertion indicator.
'on vacation' suggests removing the centre (after the inside letters have vacated the word).
'turkey' with its middle removed is 'ty'.
'went' inserted within 'ty' is 'TWENTY'.
(Other definitions for twenty that I've seen before include "One score" , "This twice denotes normal vision" , "John B Keane wrote about many young men of this age" , "No. between 5 and 1 on dartboard" , "previous clue?" .)