Party till early hours, in which I see texting double (9)
I believe the answer is:
duplicate
'double' is the definition.
(I know that double can be written as duplicate)
'party till early hours in which i see texting' is the wordplay.
'party' becomes 'dup' (Democratic Unionist Party).
'till early hours' becomes 'late' (till late at night).
'in which' is an insertion indicator.
'see texting' becomes 'c' (abbreviation of 'see' when texting).
'i'+'c'='ic'
'late' going around 'ic' is 'licate'.
'dup'+'licate'='DUPLICATE'
(Other definitions for duplicate that I've seen before include "Increase twofold" , "match" , "An exact copy" , "repeat" , "Identical to another" .)