Liked leaving university small trees (7)
I believe the answer is:
poplars
'trees' is the definition.
(poplar is a kind of tree)
'liked leaving university small' is the wordplay.
'liked' becomes 'popular' (something popular is well-liked).
'leaving' indicates named letters should be taken away.
'university' becomes 'u' (abbreviation for university).
'small' becomes 's' (abbreviation - e.g. clothes size).
'popular' with 'u' taken out is 'poplar'.
'poplar'+'s'='POPLARS'
(Another definition for poplars that I've seen is " Tall slender trees with soft yellow-ish wood".)