You and I grin broadly, imbibing wallop in tree? (9)
I believe the answer is:
whitebeam
'tree?' is the definition.
(I've seen this before)
'you and i grin broadly imbibing wallop' is the wordplay.
'you and i' becomes 'we' (we means you and I).
'grin broadly' becomes 'beam' (beam can mean to give a big smile).
'imbibing' indicates putting letters inside (imbibe can mean to drink or absorb).
'wallop' becomes 'hit' (I've seen this in another clue).
'we'+'beam'='webeam'
'webeam' going around 'hit' is 'WHITEBEAM'.
'in' acts as a link.
(Another definition for whitebeam that I've seen is " Tree related to the rowan".)