X may be the one regularly going missing (3)
I believe the answer is:
ten
'x' is the definition.
(synonyms)
'the one regularly going missing' is the wordplay.
'regularly going missing' indicates alternate letters.
'the'+'one'='theone'
The alternate letters of 'theone' are 'TEN'.
'may be' is the link.
(Other definitions for ten that I've seen before include "No of sides of a decagon" , "PM's address, Downing St" , "Roman numeral X" , "X-WORD" , "How many" .)