Sentimental old you, reflecting on game! (5)
I believe the answer is:
'sentimental' is the definition.
'old you reflecting on game' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'you' becomes 'ye' (historical version of 'you').
'reflecting' shows that the letters should be reversed in order.
'on' says to put letters next to each other.
'game' becomes 'go' (go is a board game).
'ye' backwards is 'ey'.
'oey' after 'go' is 'GOOEY'.
(Other definitions for gooey that I've seen before include "Sticky" , "Mawkish" .)