One finishes sentence? Party time! (3)
I believe the answer is:
dot
'one finishes sentence?' is the definition.
(as in a full stop)
'party time' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'time' becomes 't' (abbreviation).
'do'+'t'='DOT'
(Other definitions for dot that I've seen before include "Full stop, point" , "Shorter of the two signals used in Morse code" , "Marriage portion; spot" , "Spot, speck" , "Small round spot" .)