Taller variety of trefoil (7)
I believe the answer is:
loftier
'taller' is the definition.
('lofty' can be a synonym of 'tall')
'variety of trefoil' is the wordplay.
'variety of' indicates anagramming the letters.
'trefoil' with letters rearranged gives 'LOFTIER'.
(Other definitions for loftier that I've seen before include "Trefoil (anag.)" , "less humble" , "More exalted" , "Higher (ambitions)" .)