Game that’s small to expand a tiny bit (5)
I believe the answer is:
whist
'game' is the definition.
(whist is a card game)
'small to expand a tiny bit' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'to expand' means one lot of letters goes inside another (one word increases the size of another).
'a tiny bit' becomes 'whit' (as in 'not a whit').
's' placed inside 'whit' is 'WHIST'.
'that's' acts as a link.
(Other definitions for whist that I've seen before include "Popular card game, sometimes solo" , "Old card game" , "Card-game" , "18th-century card game, the forerunner of bridge" , "calls for silence?" .)